Velvet actually can be from several different fibers -- silk being the most expensive. A lot of the newer velvet is made from polyester, nylon, acetate, viscose as well as cotton. The difference between velvet and velveteen is if the pile is created in the vertical yarns (warp -- this is velvet) or fill yarns (weft -- velveteen). Some of the newer velvets are washable (I have a skirt and jacket that I just throw in the wash).
I would wash the velvets that you have in cool water, gentle cycle and dry them cool and keep the ones that look nice. Make sure you have some kind of color catcher if you have strong colors since the color will probably leech out.
Be warned, some of the velvets ravel and some don't -- I would staystitch with a small stitch (tighter than the normal piecing stitch) the ones that ravel. Velvet can be a sun of a gun to sew because the really nice ones on slick on the back and they want to walk. If I rubbed the velvet up and down along the seam line, the pile sort of nested -- if you pin, keep the pins in the seam area and use 1/2" seam. The slick back may wiggle, so keep a good hold of it and use a teflon or roller foot if you have them.
If you don't have or don't want to invest in a needle board, put the velvet face down on a thick towel. You want to steam it.