Wings,
Montague is right about the possible glowplug problems, but quite frankly, I haven't had any.
Yes, sometimes a few bubbles are O.K., as long as the engine runs steady, just go with the richest setting you can and still maintain a good 2 cycle, as opposed to a rough or 4 cycle sound (generaly it won't have enough power to take off if it's 4 cycling).
I've used many different foams, and while I've read that some are better than others, they all work O.K. We shot a video, with exploding planes, ect. and in those "disposable planes" we isolated the tank with wadded up news paper, and IT worked fine.

Jetts