Hi, Susie!
I don't see why you couldn't mix or layer different types of bases. Normally I use the clear and white bases when I layer, but I would think you could layer, say, goats milk and honey bases so long as the preceding layer is still warm but has a thick enough skin to support the next layer, and you spritz with rubbing alcohol between pours. I have heard that you should never try to layer different
brands of bases (i.e., bases from different manufacturers) because they are formulated differently from each other and may set up at different rates, making adhesion difficult. So you wouldn't want to layer BB's base with a craft store base, for instance.
When I use the special bases, like olive oil or goats milk, etc., I tend to stick with that base all on its own, but that's just a personal preference. You can always try to layer them up in a small batch. The nice thing is, if it doesn't work out, you can just remelt!
I've never tried to add titanium dioxide to a base. If I want white, I usually just get the white base. Or, if I want a touch or opaqueness in my clear base, I usually just melt a bit of white base in with my clear. If you add TD to a base like olive oil or honey, I'm not sure if you'd get a true, bright white since those bases have a natural golden tint to them, but it should give you some degree of opaqueness.
LCP base (like cold-process) contains less glycerin than the regular base so that it sweats less (I think I'd still wrap it in plastic just to be safe - I live in humid Florida). I used LCP once when I couldn't get the regular, and I liked it fine (although I prefer the regular base). I was afraid to try to layer LCP with the regular base, though, because I was worried they might wear down at different rates.
If you want to try just a little bit of a bunch of different bases, you might like BB's M&P base sampler kit:
http://www.brambleberry.com/A-Melt-And- ... P4280.aspxBB does have the best bases I have used. When I started out with M&P, I bought a kit at my local craft store, just to see if I liked it. The difference between the bases (and colorants and fragrance oils) was huge - BB has such high quality ingredients, stuff you can't find in the craft stores.
Have fun experimenting!