Ahead? Yeah, right. That's what I thought at the beginning of this mini-obsession. Although I am ahead, it's not quite the gap I had anticipated. Since January, I have spent about $852 on a lamp, supplies, and of course, colours (and I want more...). I probably have enough stuff for another few hundred manis. My niece and I have done about 25 manicures and 12 pedicures in that time and at $25 and $35 each (for Shellac manis and pedis) that would be $1045 plus tips, parking, etc. For me, that's more than okay because I really enjoy doing my own nails and I love the flawless nails I have with Shellac/Gelish.
I have done the reverse of what you describe and it's worked well. I have done Gelish and Shellac base and top with RCM colours. And since you ask about what goes on top, I suspect you're talking about layering, not mixing. You certainly can mix colours to create new ones, but mixing a glitter into a cream produces a color change and a very subtle effect, if any at all. It takes a lot of glitter mixed in to alter the appearance of a cream. Layering colors on top of each other creates not only new colours, but new effects. In that case, the effect (sparkle, glitter, holo, shimmer) usually goes on top for the most impact. I like RCM Glitterazzi.