Remember, you must apply decals to a glossy surface. So you do not want to apply a dull coat until after you finish applying the decals. Decal solvent as opposed to setting solution is indeed quite strong and can affect some paints (although, not all). Try using setting solution instead of solvent. Use solvent only if absolutely needed. However, I know folks who use decal solvent over future and say it works. Also, I believe Tamiya clears are acrylics, and should be okay over future. You might try a test by painting a piece of scrap plastic, putting future over it, and then trying the clear flat over that. Just allow the future to dry a day or two.