Future over Tamiya acrylics will work just fine. I do it all the time and have not had any problems. If you have a flat finish, let your paint dry for about 24 hours and then rub it down with a coffee filter. That will smooth the paint some and the Future will go on smoother.
If you do have to clean the Future off with Windex, make certain that your paint has thoroughly cured (as in several days). Since both the base paint and the Future are both acrylic, what dissolves the Future can also dissolve the base paint. I haven't had any real problems with Windex taking Tamiya acrylics off, except for flat black. I detest Tamiya flat black, and refuse to use it. ANYTHING will cause it to dissolve.