canuck
Rifleman
The SOCOM and Beowulf use factory standard 5.56 mags. Beowulf suggests that you bend the feed lips out slightly, but I never have, and haven't had a problem. I actually use Pmags for it without issues.
They are all equally compatible with the lower. I guess it depends what you want to do with it. The beowulf is ballistically similar to the .45-70. With the right load, it's good for hogs, deer and moose- it's a thumper.
I hear the Grendal is a good distance round (don't know... don't have one). What are you going to do with it?
If you're looking for "practical", stick with 5.56. Most of the ammo you've listed (except the 6.8SPC & 7.62) is expensive and/or hard to get. Since Nov. 4, Beowulf ammo that used to sell for $25 a box and was readily available, is getting upwards of $70 a box on gunbroker and hard to find.
Grendel is comparatively cheap at $10/box delivered. Bill alexander was smart when he released the Grendel - he had Prvi load a couple million rounds. No one has really jumped on the beo wagon - too bad about that. The Grendel ammo is marketed under the wolf gold line but is made by Prvi in serbia. Also a grendel mag holds 25 rounds.