They are all intermixable. On rare occasions, you’ll run into a slight difficulty. For example, a pthalo green from a top manufacturer will be highly pigmented; if you mix it with an ochre from a student line, you’ll probably find that a little of the green goes a LONG way.

Some student paints feel waxy or plastick-y. But they will still intermix with the best paints.

You can mix anything you want.

Just because a brand uses more pigment doesn’t mean it’s therefore better, it just depends on preference. Underbinding paint and stuffing paint tubes full of pigment is a recent thing, paint from the 15th through the 18th century was incredibly oily and low on pigment, they didn’t have industrial dispersion machines until the industrial revolution.

Skip the cost of Old Holland…they aren’t what they use to be! Williamsburg is excellent and my most used brand. I have at least 8 different brands that I use, but primarily WB. Gamblin has some really lovely colors. I am no longer buying WN, they have changed as well. Pigment load and quality are important to me. I mix different brands and have never had an issue…I do stay away from student grade because of too many added fillers. Gamblin isn’t student grade, so you would have quality paint at a lower cost. Watch for the “sales” on these paints at Jerry’s (often getting free shipping and no tax outside of NC). Wishing you the best in your painting adventure.