Yeah, blutack can be usefull. I also sometimes use wet tissue or aluminum foil to mask large areas instead of taping everything.
For Tamiya paint and primer, they're chemically similar enough that in theory you can start spraying color coats as soon as you've finished inspecting the final application of primer- say half an hour or so. Personally, even when I'm spraying 'compatable' mediums, I like to leave the primer to outgas at least overnight. It takes some discipline not to start spraying body color immediately- especially when the weather's perfect for painting. But time and patience truely are as crucial in body painting as paint and primer. Knowing I'm not going to start paint until tomorrow gives me the time to make sure the primer really is perfect. Primer, paint, decals, clearcoat- as a rule, I never apply more than one of these to a body per day.