I am not saying you are wrong, cause I have had first hand experience with G&G/Classic Army having trouble with certain mags, but just thought I would ask. Reason being, just recently I found some MAG brand mid caps to work in a G&G very well, unlike before.
I have no way of trying any brand without purchasing them online. I know that the G&G and CA have the same cut out on the top of the magazine. No other brand I have seen has that same notch. And everything I have read on the G&G m14 says that the CA mags are the only ones that will fit. Made sense to me because of the shape. I can afford to spend the money on the mags, if the site is legitimate. But buying random mags to try them out and having to deal with paying shipping twice and the hassle of returning them isn't worth the money or the effort to me.
Long story short, I know G&G and CA mags fit and thats easiest and cheapest.