Are you just wanting to use a deck that favors Bouffalant or are you specifically wanting to add something else to a Quad-style deck?
For the latter, you are very constrained. Ditto can help reduce the likelihood of mulliganing but is itself at risk of being donked by the fastest, strongest decks. Most of the other very good Colorless Type Pokémon are, as you already implied, Pokémon-EX.
If remaining pure Colorless is not a concern, I would considering adding some of the new Sigilyph and some of the older, Safeguard version. With Silver Mirror and Silver Bangle, you'll be in a good position to crush Pokémon-EX and Team Plasma Pokémon, at least if you can find a build that sets up fast enough. Throw in some copies of Enhanced Hammer and the only deck you lack significant counters for would be one that uses Evolutions but doesn't use Special Energy. Again, pure speculation on my part.