I've learned the hard way that even after clear coating decals, taping over them can, and often does pull up that part of the decal. I suspect that the adhesive just pulls up on everything it comes in contact with, and decals are a lot more delicate then paint and primer.
Decaling is almost always left till all the painting and a clearcoat base for decaling has been applied. Then you should seal the decals with a few coats of clear gloss.
I've never had to specifically seal the edges of Tamiya tape. Just make sure that the 1st coat is a mist tack coat that will also seal the tape which is the 1st part of the painting process.
As for painting over Future/Pledge, yes you can as with any clearcoat.