oops - sorry - didn't answer your questions..........
Tamiya clear will have to be thinned - I use isopropyl alcohol.
The Tamiya Flat Base (I assume that's what you're talking about) is actually a flat base to add to gloss paints to make them semi gloss or flat, depending on how much you add. Whatever you do, DO NOT thin the flay base by itself and apply to your model - your model will go completely flat and completely white - there's the voice of experience talkin here.
Like I said, I'd just forget about other clearcoats and just use Future. It's cheap, does a fantastic job, and there are absolutely no bad things you can say about Future. Dip your canopies in it, paint it over decals, add a little black or brown to thinned Future for washes.............it's great stuff.
LeeTree