The new 10 rounds mags from what I understand are not Service Rifle legal, however the older franken (riveted) mags are.
I'd like to see BM make their 10 round mags so they look like the old 20's. They would fit in the 20 round mag pouches (the current ones dont because of the bend, and are too short for the 30 round pouches) and would be DCM legal.
As for your single shot problem that is easily fixed! Get one of those cheap USA or PRO-Mag POS steel magazines (make sure it is the type with the over-insertion protection - the dimple over the mag catch hole) and use one of the single shot followers in it. Most of the 20 round steel magazines I have look just like the USGI 20 round magazine so they should be legal. You won't have to ruin a good USGI 20 round magazine and you have a use for those cheap & useless after market magazines.