Caineghis is definitely someone who should obviously be scaled against joining time, which is literally the endgame anyway, so not sure I'm on board with "invincible early." Granted he'll ROFLstomp the early floors anyway just because the earlier floors are easier and are unlikely to have powerful, accurate fire mages, so.
Not really a fan of Fogel in the dungeon, he's a boring pile of high stats + OB interp headaches. Cain would at least be an interesting alternative, and has some interesting features, like fire weakness + counters. Only possible worry: Firefly Sealstone is *busted* on Cain, since anyone non-ranged throws themselves into the wood chipper basically with their first attack. I suppose Firefly / Eph and Firefly / FF1 Knight already exist, but Cain is unusually good at the whole not-dying thing. Then throw in Firefly Cain + Aika/Chris/other magic hosers, and have fun as long as Cain isn't fighting, uh, Haste / Enfired Sazh or something weird like that.