I've been using multiple 10ml syringes to mix up paint in small batches, it works really well as its easy to measure and make small adjustments to the quantity of paint.

However each syringe generally only lasts 4 plunges before the friction builds up on the sides of it and causes the rubber to pull off, but at <$1 each its not that bad! I mix it up in small glass jars just by shaking them for a bit

as stated, your probably best to mix up paints of the same brand/type to ensure compatibility. i've been using brightside paint and it mixes fine, and i've nearly made up my mind as to what colour i'm going to do after about 20 different variations!

-Evan