First thing you should do when you get a chance is read the FAQ section about clearcoats. To answer you though, a lot of factors come into play regarding decals and clearcoat such as the type of clear, maker of the decals, and how the clear is applied. Tamiya clear is excellent but you have to be very careful about which decals you use under their(Tamiya) clear. Decals should be dry for at least a week before you clear them (I wait that long). Usuallythe first coats of clear should be misted over the decals in order to minimize the chance of them being eaten by the clear. let the first few mist coats dry for a couple of days then you can try to cover them with regular coats.
I'm no professional but this is the method I use for my models. Take care!!