I just tested it out, and you're not all that limited. The only thing that can cause problems is the D-Pad, but that can be solved by keeping the finger holding select in the right place.
I would have it that the faster you enter the command the more of a boost the attack gets, and the complexity of the command depends on the GF, i.e. early GFs like Shiva and Ifrit have fairly simple commands (but still more challenging to enter than Zell's non-finisher Duel commands), but GFs like Eden would have more complex commands.