With Siphon feed, I would usually thin in the cup or bottle, but with gravity feed I pre mix and thin in separate bottles. I found 17ml eyedropper bottles on eBay from overseas that were something like $8 for 50 shipped. Best AB investment I can think of.

So, yeah, you could paint anything with gravity feed, they come in all different sized cups, it's just a matter of how many times you want to fill it. With me, the more that's in the cup, the more to clean and more to spill.