Tamiya p-51 mustang

While I have not used that particular paint on anything, I don't know any that is not a gloss paint that does not need a gloss coat before the decals. I would recommend the gloss coat.  A gloss sealer coat after the decals and before the weathering is good too.  It helps keep the weathering for building up around the decal edge.  The en the final coat would be the only question... How shiny should it be?

As far as how shiny the finished bird should be.... if you have a reference photo of the a/c you are building, use that as a reference.  Otherwise, an NMF a/c could be anywhere between shiny to down right dull. And somewhere in the middle of those, I have seen pics of showing some dirtier a/c but still have a shine showing through. Depends on a lot of factors like where was it stationed and how well was it maintained. 

Ant wrote:

should i fit the decals then glosscote or gloss it then decal and gloss again.

If the original basecoat is glossy enough, you can skip the initial gloss step.  If you have any questions about whether the base paint is glossy enough for decals, it's better to be safe than sorry, and gloss before applying decals.  After the decals have been set for a day, and before applying any clear, wipe them with a damp cloth to remove any decal setting solution residue.  (I've heard that if you omit this step, that years later stains may appear where the decal setting solution had dried.)  Prior to flat coating, apply another gloss coat over the decals.  If you skip that final gloss coat, the flat coat may look different on the decals than it does on the rest of the model.  By glossing everything, the flat fininsh will be consistent over the entire surface.