Levitate should stay on the line as the only ability.
I'm not seeing Koffing/Weezing getting Venom Strike despite being poison; it doesn't really seem like it could do that. On the other hand, Weezing actually does have a surprisingly solid movepool, with moves such as Fire Blast, Thunderbolt, and Shadow Ball all being TMs. Perhaps Umbrage could make its way in as a TM?
Just some preliminary thoughts to consider.