ErikC12 wrote:1) vallejo model color is specifally made for brush painting right? vallejo model air is basically for airbrushing (which I wont be doing just yet)
That's correct, though you can also use Vallejo model colour for airbrushing if you dilute it. Model air comes pre-diluted.
2) the vallejo mediums (matte, gloss...) improve the flow of the acrylic paint and help prevent brush strokes?
Maybe a bit, but probably no more than water, matte and gloss are primarily to give the paint a different finish. Gloss will have a levelling agent that makes it look smooth and shiny, but you can still end up with shiny brush strokes if you're not careful. You should also be aware that after you paint you will probably want to varnish, which will change whatever finish is on the paint.
3) drying retarder slows down the fast drying of acrylic paint so it can be handled longer?
Correct, which actually does make it good for preventing brush strokes and doing fine detail painting.
4) are you supposed to use the medium and retarder together? or is it one or the other... and in what ratio's? paint to retarder and paint to medium or is that basically experiment on a piece of scrap till you find a good mix?
You can use them both together. The ratios will depend on what effect you are trying to get, and things that can't really be accounted for like the humidity levels in your room etc... So yeah, it's basically a case of experiment until you get what you are looking for. If you work from a palette you can usually tell when the paint is the right consistency.