I have only ever nebbed Colistin.
I already nebbed that before TOBI existed and when it did my doctor thought it best not to neb it.
This is because my cultures showed me sensitive to only a few antibiotics. Tobramycine, Ceftazidime and Colimycine.
With flare-ups I always got Tobra/Cefta and and was always nebbing Colistin. Colistin was known for the fact that you don't get resistent to it very easily when nebbing so you can neb it for years and years and years. However with TOBI my doctor was afraid I would become resistent to that with the result that we didn't have my IV combo at hand anymore, with the flare-up.
Colistin does foam. Be sure when you mix it not to shake the vial but sort of twist it and don't put the saline with the powder with too much force. You can also make it in advance, before nebbing Pulmozyme, HS, Albuterol or whatever or just a little before you want to do your nebbing