I'm sorry if this is a repeat, but the search function isn't giving me anything. I have an Anderson upper that HATES polymer mags, you literally have to punch in a Magpul. But it takes GI mags and Mako mags like butter. Is there a way I can grind down the mag well to accept the MP mags or should I just stock GI mags?

I have an old Colt with a tight mag well. Just use aluminum mags in that one.
Don’t recall if it likes a lancer mags but it does not like PMags.
Look at the mags to see where they rub.
Gen 2 or 3 the issue?

Curious about this as well. Gen2's seem to have been notoriously tight for many, and there were a shitton of them that got bundled sometimes with ammo deals.

FWIW, if I wanted to use them I'd rather "work" the mags in (yes, it's tedious) than go grinding on the upper.

Dump the offender/s.

I don't dick around with cherry picking mags and lowers personally. If all mags/lowers do not play nice something gets shit canned.

This a bolt-closed issue or bolt-open issue or both?

If'n you are gonna go grinding on anything, always grind on the cheapest part.  I'd mark up the mags with some type of indicator that will scrape/rub off and insert and remove a few times and see where the rub marks are, then sand on the mags in that area a very little, rinse and repeat.

I'd also just switch to quality metal mags.  No price difference, less issues.  I don't have a pile of mags like some, but I do have "enough" aluminum mags for each rifle and maybe 15 total p-mags "just because" either to neckbeard, trade, or as it turns out for me (first use of 'em in 3 years!) converting a couple with the Endomag insert to run in a 9mm.