I actually agree with this and this was phrased in a much more coherent way than I could have phrased it myself! I think 10 as a franchise cap would be a very reasonable number while still keeping the 6 per cast for standalone canons is equally fair. As mentioned above, franchises like Tales and Persona and even Final Fantasy have some minor continuity and overlap but I also agree they're different enough and separated enough that I feel they would be their own separate canons. But for things that have a continuity that spreads to an extended universe it could be overwhelming to allow 6 characters for every individual game/movie/whatever especially for what is meant to be a capped and smaller game.