That wasn't a heck of a long time to steep them. That MIGHT be the culprit. Plus your grains are sitting in an ideal temperature for an even shorter time. My $0.02 would be to go ahead and heat your water to 165 or so, pop your cold grains in and let it steep (or some process like that)...
Don't worry about it. The amount is small and there's not a brilliant way to prevent this. If you're using the muslin bag, you can get a nylon bag with finer mesh at the homebrew store. I prefer those and they're reusable.
This sounds like a terribly fun experiment. From my perspective, I was taught to brew, so wouldn't try a lot of the stuff you are. Be sure to post anything that works please!
Sour beers you tried were probably lambics?
How different are the beers you're brewing, in terms of gravity and such? Just thinking out loud (or in type), and maybe it's the dissolved stuff affecting things?
The one advantage to transferring to the carboy is you can use a giant tube for blowoff. We put high OG beers in carboys for primary for this reason. Harder to clean, but you don't blow your lid off.