Going with Large and grouping them does seem the best way to go.
Let's work on ability scores. Here are some from other Large oozes:
White Pudding: Str 15, Dex 2, Con 20, Int —, Wis 1, Cha 1
Creeping Stone: Str 18, Dex 1, Con 22, Int -, Wis 1, Cha 1
Shadow Jelly: Str 15, Dex 1, Con 18, Int -, Wis 1, Cha 1
Lava Ooze: Str 19, Dex 1, Con 24, Int -, Wis 1, Cha 1
Void Ooze: Str 20, Dex 1, Con 26, Int -, Wis 1, Cha 1
These guys have Int 6 per the original stat block, so we'll probably want to increase the other mental stats as well.