I think its just Basil who is the drug addict here with Lavender is more about his ability poisoning his opponent which I doubt that will hurt Boo at all.
Buu's full power seemed to be quite a bit above Basil if I remembered correctly. I can't imagine Lavender's power working well, if at all on Buu, I'd definitely give this to Buu.
Fit Boo fought on par with base Goku, no? I'd give it to Lavender and Basil as a duo since they were at least on par with base Galu, forcing him and Veggie to use SS.