You are, as far as I know, right. The difference between 1 cus\3 ori vs 4 ori is neglible. Plus, custom characters have their unique dialogue and voice actors too. It's how I like to play the game.
I believe Fane, Sebille, and Beast's story quests are bugged with the steam achievements, but beyond that all of them are playable if you don't mind that (or got in GOG heyo).
You're green to go. Personal recommendation though; don't pick Fane and Red Prince in the same party. They're both smarmy arrogant jerks and it's not exactly diverse, despite them both being individually interesting.
