Hybrids can come about if you put sternotherus of different species together. The odoratus & carinatus may need deeper water then the muds. Mainly if you keep same sex(females) individuals & visual barriers then all should be fine in that size tank. Just make sure you have plenty of places they can climb to the surface & keep from getting wedged in places. I have 2.3 carinatus in a 75gal. tank. Also 1.1 odoratus & a pelomedusa in a 55. I have 1.2 muds in a 55 gal rubbermaid. You should be fine with up to 7 individuals. If you don't want to breed then get females. Males have huge bulges in their tail base. females are short streamlined tails