It'd also be possible to make the poison weapons not suck, if that's an issue, or have it be a semi-trivial added property to a certain (type of) special/unique weapon - my first idea about the E-mouse Assassin promote item thing was to bring out samples of some sort of deadly poison, assuming something like that was used for Assassin promotion. Changing that reward to a unique or earlier-in-the-game-than-it-should-be poisoned sword (or several of them!) would keep that theme, justify a poison weapon, and work regardless of the temptation to unify the promotion seal collection.
I'm afraid I'm not eager to budge on my self-insert's character type and stats (well, maybe a base SPD and 5% to put elsewhere or for that extra MOV), but I could throw in an OC-like tanky character or two, if that's wanted... how many playables are there to work with, typically, and how close are we to that number?