Steve, Tamiya Flat Base isn't really paint. It is basically the carrier that is used to create flat paint, but it does have a slight white tint to it. I personally mix it with Future to get a flat finish. Both XF-21 and Future are acrylics so there is no problem in mixing them. About 3 to 4 parts Future to 1 part XF-21 will give a flat finish. Five or 6 parts Future to 1 part XF-21 will give a semi-gloss finish. The higher the ratio of XF-21 to Future the less clear it is (a very slight white haze to it). Take a look at the F-105 on my models page (the link is in my signature). It was sprayed with XF-21 + Future at about 4:1.
I've only tried hand brushing it on very small areas, so I don't know how well that will work. I normally airbrush it at about 10 psi or so in light coats. I let it dry about 15 minutes between coats and then spray another light coat until I get the finish I like.
I forgot to mention that you might be able to get the XF-21 off your decals using some Windex on a Q-tip. I don't know how it will affect the decals, so I'd recommend you stick on on some scrap, and try it there first.