Regarding worms, I think the touchscreen controls are beyond annoying, which is why I downloaded the psx version, but i think that if the worms were tweaked so to speak, then it would be great fun, especially if they added network play.
Regarding ff games, i've played them all from 3-13 and loved them all. 8 is my favourite actually but I understand the hate towards it from most people. You if you liked 7, you'll likely like 9... also 1-6 are nes/snes games and as such are tiny files (like 1-3mb) so I think they're well worth getting. Incidentally, the gba versions of 4-6 are regarded as the best, and are worth getting if you want to play them mobile, rather than a big screen where the snes version is your best bet. Avoid the psx version as it has by far the longest load times even when emulated. funnily enough I couldn't sleep last night and I ranked the ff games from best to worst in my head (yes I am that sad) it goes 8>7>10>9>5>12>4>13>6.
These emulators do work without buttons (on screen buttons), but your phone must support multi touch (being able to sense multiple touches at the same time) and to be honest, I think that they are not playable in that way. (the gba maybe as it has a total of 8 buttons including the d-pad whereas the snes and psx have 12 and 14 respectively)
I only have a 16gb card on in my phone and psx games can be anything between 100-700 mb (ff games have multiple discs so ff9 actually edges on 2.1gb all together)also worth taking into consideration