AltTank - welcome to the forums!
Somehow I can't see the picture you're talking 'bout. As for your question - the general opinnion is that you should put the decals on glossy surface for best results, so putting a gloss coat at least on the general area where the decals go would help you. Mind you, if you make a glossy "spot" with visible edges on a model it might show from underneath even several layers of whatever you put on top of it. So the gloss area should either have feathered edges, or those edges should align with the edges of surrounding parts, then it might look OK. Or you gloss the whole model, and then put the decals on, some more gloss and then the flat coat on top of that - that gets all your bases covered.
Hope it helps, have a nice day
Paweł