I use Mr Color lacquers. I haven't found a clear-coat is needed before applying decals over Mr Color because they go down so smoothly. To be sure, you do need a clear-coat atop the decals to ensure they are protected and fixed in place. You especially need a gloss coat over decals if you're going to apply weathering. As you noted, you want to make sure the gloss coat beneath weathering finishes isn't affected by the weathering media. As I've been using Tamiya and Vallejo washes, Mr Color C33 clear works just fine.
In addition to Mr Color C33 for gloss coats, I use Microscale Industries Micro Flat and Micro Satin respectively for the final flat and satin finishes; these thin with water. That's exactly how the models in my current signature were completed.
If you don't have a really smooth finish direct from Mr Color, or Alclad for that matter, you have a surface preparation or paint application problem. A gloss coat should fix that for you this time.
Lacquer paints provide the thinnest paint coat. Enamels provide a medium thickness paint coat, while acrylics are thickest--too thick IMHO. With that, surface preparation is critically important for lacquers. I use 6-cut Swiss Pattern needle files and polishing pads (up to 12000 grit) to get the smoothest possible plastic finish. Once I'm satisfied with the plastic surface, I'll give the model a wash-down with 91% Isopropyl Alcohol applied with cotton pads and cotton buds to ensure a clean and pristine paint surface.
I thin Mr Color, and all other paints, to the consistency of 1% milk using the paint's airbrush thinner. For Mr Color, I use Mr Leveling Thinner. I spray very thin coats at 1 atm (~15 psi). Adjust this pressure while air is blowing out the airbrush at full volume. I spray at distances between 0.25 to 2 inches. Use a grazing light when you paint so that you can see the paint hitting the surface. It should be wet when it hits, and dry quickly after that. So quickly that you can apply multiple coats in one session. If you want a gloss paint finish, apply thin coats until you have coverage and then apply a final wetter gloss coat. This is especially important for gloss white and yellow. Don't try to blast on a gloss color coat in one pass!!! Before color paint, I prime with thinned Mr Surfacer 1200 applied exactly as described above. This provides a beautifully smooth finish that hides slight surface scratches.
I use a 0.5mm nozzle for primers, base colors, clear coats, and Alclad or other metallics. I use a 0.3mm nozzle for detail work.