Exactly. Succubi are infiltrators. Thats how they hunt. They are meant to blend in with their prey. Now if you want to have the ones laying down be on 40mm bases and the rest on 28/30mm bases thats fine. But dont make them all 40mm.
40mm bases are the bane of role players. Its not a base size thats supported very well outside of wargaming. It plays havoc with the line of sight rules as they work with a grid or hex style of play. If you play grid less its not so bad but I dont really know to many games that run grid less.