Such things happen often. xD So, we're in a position of making a plot of some kind, and of course, the characters included in set RP. Now, I would assume all characters that hold a role in their games would have some kind of role here. That... would be a huge cast. On average, FE has a cast of allied characters that can go upwards to about 30+ in just the main story, though this does not include post-game/DLC characters (usually bosses you face in the main story or from other FE games) one can acquire, which could potentially spike the number to around 40-50, maybe 60, depending on the game (and yes, they ALL have names and a role of some kind). This also comes without saying that after a certain chapter, or the option that characters leave the battlefield heavily, but not fatally, wounded is set to off, characters in FE, if one plays a bit too recklessly, can die permanently, and cannot be used again after they fall.
Final Fantasy has parties of 4-9, depending on the game (tactics was an army game, so could have an army of 30 characters, tops), and if they fall in combat, a Phoenix Down (or Phoenix Pinion and/or Life/Revive spells) can be used by a comrade to revive those characters, albeit with low stamina.
Both do have class systems of some kind, but there are different ways of doing it. Promoting a character in FE requires a 'Seal' item of some kind, unless they are a Trainee-rank character. I don't know how it works in DS titles and beyond, but to say the least, it requires a special item to promote a character to the top tier class, a specific kind, though, depending on the current class (Knights require 'Knight Seals' in order to promote into Heavy Knight or General, for example. 'Master Seal''s, rare as they are, allow the promotion of any unit as long as the requirement is met). However, characters could only attack or heal, depending on the class, and on occasion, have an undodgable, unblockable critical attack that deals triple base damage that can usually 1HKO foes (think of Marth's 'Critical Hit' final smash in brawl). Relationships with certain characters help improve their latent abilities if they were close to one another, ranging from offensive to defensive in nature, and usually improves critical rate with those characters as well.
Final Fantasy required specific training in one or multiple classes to a certain degree in order to promote to other classes, though sometimes, other conditions, such as acquiring crystal fragments or awakening crystals, often allow class changes. FFI required a badge of courage to be shown to Bahamut in order to promote into better classes. Otherwise, from FF6 onwards, you could effectively customize magic or physical output with all characters, depending on the gear equipped. FF6 with Magicite, 7 with Materia, 8 with Guardian Forces and the augment system, and 13 with the crystarium/paradigm shift customization. Some games, however, kind of keep the characters to a single class, and pieces of equipment help give them new powers, abilities, and passive skills instead, and get powerful 'limit' commands or new combat forms like Trance from constant beatings from baddies, which may give access to new powers and abilities that cannot be accessed normally.
I do apologize if I'm making this sound more complicated than it should... but, mostly, I'm also trying to help kind of explain how things work between games to help those who may not know so they will have a general understanding, the differences they yield, and how one could attempt to blend them both in the RP. That, or remove one system and stick with the other, idk. As this RP is blending both the worlds into one; maybe some rules of FF remain, while some rules of FE remain. Though, FE does run on an army system, whereas FF usually works on a party system (some FF games, like Tactics and the MMOs, to a degree, go the army path). Other than that, this is not my RP; I'm just providing info for Alura so they may figure something out with the possibilities and points I have listed.