So... where are Pichu and Raichu? O.o
I thought plusle and minun were closer to pichu, but this is fine.
I was going to do it that way, but then I looked up the stats - Minun and Plusle actually have much stronger stats than Pikachu :/
You don't say. Regardless, I think the common ancestor is too big for all of them but pikachu.
That's what Coolsteph said, but Pikachu is so iconic that I decided the ancestor should look a bit like him. If a big Pokémon evolving into two small Pokémon is really that large of a problem, the Emolga-Pachirisu ancestor and the Plusle-Minun ancestor can both have a small-bodied shared ancestor that evolved from the Pikachu ancestor.
Sorry but just not your best. :l