Basically it comes down to how good you are about cleaning your stuff. If you're the kind of person you can count on to clean it as soon as you get back from a ride, then there's no reason not to put whatever you want in it. Chocolate pudding, ice cream.

Mold can be cleaned out, but it takes some attention. I would invest in one of the long brushes in any case. Unless you use an air compressor to dry your drink tube, it's going to get specks of mold now and then, just because it takes so long to dry. That's when you need that brush. Same with the bite valve - again, it takes so long to dry that it WILL get mold unless you run it through the denture tablets now and then.