Really it will come down to what can you get your hands on. Peacocks are available from on-line sources, but most others you have to get at a LFS or from someone who has an unwanted hitchhiker (it pays to help them out). If you have a decent LFS around you, as them if they can special order a mantis for you, and if they can get different kinds. My LFS orders a mantis, and gets a different kind every time. the best way for an ID is to look on the lurkers guide to stomatopods. If you can't find a close match there, take a few pics from different angles and post them here, Dr. Roy is a stomatostud for Cal Berkeley (I think) and he is you best resource, as there is very little published about keeping these critters. As for the tank size, most of the commonly available species would be fine in a 20g. Peacocks need a little bigger house.