GSP, Tetraodon nigroviridis, get larger - up to 6" - and need higher salinity - high brackish to full marine - and is quite teritorial/aggressive. There are no external sex differences.
Figure-8s, T. biocellatus are smaller - ~2 1/3" - will do fine in light brackish water, are less territorial or aggressive than GSP, but still pretty iffy with most other fish. No external sex differences.
Why would you feed guppies? They are eaters of inverts, not fish, in the wild.
Snails are a lot easier to breed anyway.
