You can take Inch mags and cut up the beak and they will fit metric receivers. But if you do that, God will kill a puppy. Inch mags are harder to find and expensive. Inch FAL owners will cry. Metric mags are common. Sell the Inch mags and buy a Metric if needed.
LOL!
Anyway, RFBs take Metric mags. Just because DSA or Kel-Tec is an American company has nothing to do with the FAL being metric or inch. And actually, most of the parts interchange anyway (within limits). DSA bought the Belgian FN prints iirc, and they build everything to Metric specs. Without getting into FAL minutiae there's really not enough different from a shooter's perspective to bother over, and since you are an RFB owner, all you need to know is METRIC MAGS for the RFB.
The 30-rd mags sound great in theory but heavy and are not always 100% reliable; the 20-rd mags are time tested and if you can get good surplus mags they will work. The DSA new mags are spotty at best unless they have resolved their production difficulties. If they have not, I would avoid the DSA new production US mags.
There was only one company that made plastic FAL mags (Thermold) and they are hard to find.
I bet if the RFB ever takes off, PMAG or somebody will make polymer mags for the FAL and it would be a US part, to boot. We can only hope.