Contrary to popular custom, of you research betta care in places that actually care about the fish, they are not suitable for the unheated, less than 3 gallon containers, people like to force them to live in.
If there is no benifit to either animals with this arrangement, I would not do it.
As for hermit crabs, I have kept hermit crabs in greenhouse-sized enclosures. They need their own food and salt water pools to be healthy and they like things warmer, substrate to burrow in, and other elements that aren't really suitable in an average chameleon set-up. Hermit crabs are omnivores and nocturnal. They will climb about at night, at the minimum disturbing your chameleons and at the worst, possible injuring or consuming the sleeping chameleon.
Do I think cohabitation of species is impossible? No, I do not. I think it's often frowned upon, because people fail to account for the risk of transfer of disease, conflicts between the species, inadequate space to provide for the needs of both animals, and they fail to throughly consider the needs of each animal and compare them.
Amongst things that might do well with chameleons and pose no risk to them are day geckos and anoles, but the risk of parasites is still there and the geckos and anoles are likely to fall prey to the chameleon at some point. Geckos and anoles are also much faster at consuming prey and could rob the chameleon of food, if not monitored closely. There are always risk with these arrangements and without a greenhouse-sized cage, or at the least a Dragon Strand Atrium cage and accepting the risk of the smaller lizards becoming prey, this is not really a viable scenario. I have large enclosures and I've wanted to introduce day geckos, but I'm not willing to have them consumed by my chameleons, so I haven't done so.
