... the Espers look different in FFT and FFXII? Is it canon, or graphical limitations?
creative liberty
caus the creators of XII suck balls.
I think it's more or less because the creators didn't have a solid design for them in mind yet. So when they were making FFXII, they decided to add in things that they thought would be cool that came after FFT.
If it's not canon, then no one will shit a brick if you reinterpret the FFXII-only espers when spriting them for FFT? Because I like the FFT espers better than the FFXII ones, except maybe for Ultima.
Small details like these can vary between artists, and really no one cares. Do whatever you want.