Wash directly on top of the base coat if you can. You want the wash to be able to grip a surface better so it doesn't pool. Microscopic surface imperfections help with that, especially if you airbrushed. Gloss coat after the paint for the decals to adhere better and protect the paint. If you need to wash after the decals for weathering then I suggest a flat coat over the decals and whole model for grip, then the whole wash and weathering stick, then another flat coat to seal it up.
Just consider clear coats like saving your work in a file. Gloss for paint layers and protection and decals, flat for a wash or final seal.
I usually will lay down a wash after less than an hour of the base paint being finished but I give the wash itself several hours to dry fully. Acrylic paint takes a few days to fully cure but as long as you aren't laying on the solvents, you can keep adding more paint layers to it with no problems. Big decals can pose a problem if you don't let things cure. Tiny ones, not so much.