There is always some danger when keeping multiple reptiles together in one enclosure, even when they are of the same species, due to differences of personality, and the level of aggression each one may have. That being said, males should never be housed together because they will cause bodily injury to each other, and they could fight to the point of death to one of the males.
I also think that it is important to mention that you should not house a male and a female together if you are not ready to breed or you will have breeding, you will have eggs, and you will get hatchlings. This is something that you may want to carefully consider before you decide to do it. If you do decide to do this, you will want to make sure that you have enough money, time, and space for all that is to come.
Although males should not be kept together, females can be kept together with close supervision during the first few weeks. You want to closely supervise them to make sure that everyone is eating, that no one is being bullied, and to make sure that no serious fighting occurs. Many people think that males are the only ones who are territorial but females are just as territorial and can be even more so when they are gravid and about to lay eggs. Now although I have successfully kept a tank of four female cresties together without hardly any incidents for quite a few years, there are some who just do not do well with others and may need to be housed by themselves their whole lives, just as there are some who may be fine with their cage mates for years, and then just one day they may end up becoming really aggressive towards the others in the tank with them and may need a few months apart before being re-introduced to the group at a later date. I feel it is also important to mention that i do separate breeding females from those that are not breeding. This reduces the risk of bullying on either part, and is less stressful in general for the gravid female.
I would also like to say, that although females do usually get along together, they may also try to establish some sort of pecking order among those in the cage. This is nothing to be alarmed about! Small squabbles are to be expected if two or more of the females are more dominant than others. These little fights will at most leave on or both parties with small scrapes on the nose. BUT if it is more than a small scrape, then the geckos in question should be separated immediately. They may also make noise, but that does not necessarily mean that they are fighting. I have found that my females love to "talk" to each other at night, especially when a new female is introduced!
IMPORTANT: When introducing a new animal to an existing enclosure, it is very important to thoroughly clean the cage, the plants (fake or living), any other cage furniture, and replace the substrate before you add "the new guy" to the enclosure. It is also important that you arrange the tank differently before you put the cresties back inside so that there is no remembrance of decided territories and the tank seems like it is "new" to everyone. It is a good idea to put the new Crested Gecko into the enclosure for about twenty minutes before you add the crestie/s that originally inhabited the enclosure. This way the one who is being integrated into the group has her scent all over the cage before the "original/s" are returned to the enclosure. This may seem like a lot of work, but I have done this every time, and have not had any major problems while integrating a new animal into an existing enclosure.
Although this can be done, it is not recommended to add reptiles that are not of the same species into an enclosure together unless you have a lot of experience with both of the animals that you are deciding to use. You also need to make sure that you have enough space for the animals to get completely away from each other and that both animals require the same type of environment. This means that you CAN NOT house an animal that only needs 30% humidity with and animal that needs at least 70%. This will make one of them extremely sick. The same thing goes with temperatures, you can not house an animal with a Crested Gecko who requires a heat source of at least 85*, because if you go with one temp or the other (even if you go somewhere in the middle) one of them will become extremely ill due to lack of the proper temperature.
This may leave you wondering, "Well gosh! What can I add to my Crested Gecko enclosure?!?!?" You can add African Giant Millipedes to your enclosure without any worries.
While these large "bugs" may seem a bit unsettling to some, they can be very beneficial to those who are keeping naturalistic terrariums.
These millipedes can get up to ten inches when they are fully grown. They are expected to live anywhere from seven to ten years. While these guys are supposed to secrete an irritating liquid in self defense, most owners have said that they have never personally experienced this, and if they have, it was shortly after taking them out of their shipping box. I don't know about you, but if I had just been shipped in a bumpy box, I think that I would be a little defensive as well! This being said, if this liquid gets in your eyes or your mouth it can be harmful, so make sure that you wash your hands every time you handle them, just like you must wash your hands every time you handle a reptile.
These guys will also do very well with other Millipedes, and will quite readily breed when a male and a female are housed together, so you could very likely end up with babies if keeping the opposite sex together. The temperature during the day needs to be at least at 72* with a slight drop at night. They also like to burrow, so a three to four inch layer of peat moss, or peat moss mixture is recommended for their substrate.
The cage size that is recommended for these guys is at least 15 gallons which can safely house two adults. The general rule that you want to follow is that the cage is twice the length of the millipede, and at least as wide as the millipede is long. While floor space is more important than hight, it is still important to have a lid on top of the enclosure. These guys are also herbivores, so they like to eat soft fruits, and prefer fruits that are beginning to decay, so it is alright to leave it in the cage for a day or two and calcium should be added to the diet.(lightly dust the food with calcium powder before feeding) These guys will also eat the Crested Gecko diet if it is available to them. You also want to make sure that you have a shallow water dish with clean water in it, available at all times.
These guys make great companions for Crested Geckos. Though I personally do not have one at the moment, they do not seem to show any sort of aggression towards each other and can quite peacefully live together. The Millipede seems to help keep the cage clean because it likes to eat things that are decaying, and they just seem like they would be great to house together. There are a lot of people who house the two of them together without any problems what so ever. I can not wait to get a few and see how things work out!
IMPORTANT: It is important that you do not directly spray your Millipedes with water because they not only breath through their skin, but they also have their own set of body mites. These "mites" have a symbiotic relationship with their hosts, and directly spraying your Millipede with water could wash them off. Though these little critters are called mites, they will not affect your Crested Gecko, or any other reptiles you decide to keep these guys with. These "mites" play an important role in your Millipedes health and over all well being, so do not try to rid your Milli of these little guys because it could result in the death of your Millipede.
All photos of the Millipedes in this section were taken by Sarah from Lunar Gecko