You can but remember that process inks are transparent, so with your gray shirts, that gray will effect your ink colors. You may want to add a white underbase.
You could do that to make 3 pastel spot colors. Generally when using process inks you're relying on their translucence and halftones to "blend" together to create colors, but there's nothing to prevent using them to, say, use magenta with white to create a nice pink.Thank you both for your advice. I never factored in the transparency and how it would effect the color if the shirt was not white!
Rather than printing using cmyk process, I guess what I was hoping to do was mix my colors using process plastisol ink with some regular white plastisol ink..(The colors I am printing are all pastel -green, blue and orange) I've just never used process ink and didn't know if it can be mixed with regular plastisol? I figured that using the process inks would make the ink a little softer than mixing all regular plastisol.