This will probably sound lazy, but for me it's easier to replace characters when the slots are already pre bulit so if anyone can answer thanks in advance!
Well, yes, that is extremely lazy. There are tons of Packs out there with BrawlEx. And, if you wanted to replace a character entirely, it would be a pain in the butt.
I believe the biggest pack is Nebulon's. Just search Nebulon on Brawl Vault.