I would think the little 20 rd Thermold mags would work great in that.
I read somewhere that the MVP doesn’t work well with Magpul mags — supposedly, did not properly seat them. Standard GI mags seemed to work well. I don’t know about Thermold polymer mags. I know of them, but all I have ever used were GI and Magpul mags in my ARs. I think it’s a very good design to use cheap, standard AR mags. All manufactures would be wise to follow suit.