Even though it’s taboo to cohabitate different species it can be done under certain circumstances. However, these two species are not the two that I would try it with. Firebellys are somewhat toxic as most toads are, they are likely wild caught and may harbor parasites and other pathogens and they can foul a small water feature very quickly. If you want fbts I’d say set up a separate vivarium for them and enjoy them that way. Why risk a healthy gecko ( I’ve never seen any fish scaled species for sale in the US btw).