Acrylic paints

You can always cover the flat paints with a gloss coat. 

When I want acrylic gloss, I use Tamiya or I clear coat. But there are a few other brands around. I know that Testors square bottle acrylic model paints offers gloss. But have no idea how good they might be.

I actually have used quite a bit of clear coated craft paints. Though lately I've been back on lacquers and enamels.

If you can't find Future floor polish (or whatever it is called now) for a gloss coat,  Holloway House Quick Shine - available at Lowes, Home Depot, or many big box stores makes an excellent hard gloss coat.   27 oz for less than 9 bux.

To go with a product with a model manufacturer's logo, AK Interactive Intermediate Gauzy Agent is a good acrylic gloss item.  Small bottles get more expensive.

Scale Colors Paints makes several products in Gloss, Satin, and Matte finish.   They brush (keep a wet edge) and airbrush well

These are all acrylic products.  You can mix your Scale 75 paint with them for your desired level of opacity and gloss

AK''s Gauzy Shine Enhancer or Alclad Aqua Gloss is what I use.

My favourite Acrylic finishes are Windsor & Newton Galleria, in Mat, Satin and Gloss, from the Art shops.

I find them the least problematical for me.

Tamiya X range are gloss, Tamiya XF are flat, but the if smell isn't to your liking... 

Hannants XtraAcrylics are acrylics, with a semi gloss sheen, may be hard to get outside the UK though.

Since you have invested in a range of paints, get to know then & get used to using them.

With any experiments using non-brand finishes over (or in) your paint, always experiment on a non-important model, and NEVER mix untested in your airbrush! (Vallejo + IPA/alcohol = rubbery goop!)

Broadly, most acrylics are flat, but your technique & thinning/additive mediums will affect the finish.

You can add gloss/semi gloss to acrylics to increase shine, at the expense of opacity, sometines to get that translucent effect on purpose.

I have used Future with Badger StyNylRes primer, it gave a nice sheen, and hardened the finish against handling as well.

There is a difference between marker peens and paint pens.  I find both are useful.  Some marking pens put down a neat transparent color on glossy material.