1) We have art, thankfully, as mentioned.
2) This is the worst part for me. We don't really have characters, nor anything major to throw them in. If we ranked the 4 PCs, then I'd personally see them as...well, generic Onion Knights, and since they don't have unique, storebought equipment, they're stuck with...being unable to break defense, and being beyond Puny, I think? Worse than FF5 - they at least have characterization (albeit not amazing), and reasoning to have classes from the crystals. I'd say this game should get the current FF5 treatment, and just have plot-important bosses ranked.
3) As for the ranking thing! Rank Cloud of Darkness, Zande, Dorga, Unne, no others. Those bosses are all plot-important, the only plot-importants IIRC. Do not really support PCs, due to...no plot (FF3o only for me at the moment - they have plot in FF3DS, so...I'd probably support them in the end, but...well, Punies, likely)
4) Eh, formsplit is normal for most games. See above, mostly, for plot stuff. Combat-wise...eh. It happens - it's going to happen no matter what with any game with split issues.
5) None, to me. Do not rank random classes. Rank CHARACTERS (this was my issue with FF1 and PKMN...and still is, to an extent).
I'd say those are the primary issues, really. Otherwise works in the DL for all other reasons (fits format for rankability, drawing, etc.)