NEMA 14-50 outlet installation - uninsulated ground ok to use?

Bare ground wire is fine almost everywhere.

Pools and spas are an exception if I recall correctly.

This is not a pool or spa. It's fine.

Grounding

Actually, the ground wire needs to go to the metal box first.

The 14-50 receptacle will pick up its ground via hard-flush bare-metal face contact between the yoke of the receptacle and the metal box.

Just make sure the surfaces are clean of paint or rust, and any of those little square "screw keepers" are removed.

You can hardwire a ground from the 14-30 recep to a pigtail coming up from the metal box if you really want to, but it's unnecessary.

Aluminum (don't panic)

Oh, one other thing. That ground wire may be tinned copper wire, but it might also be aluminum. There's nothing wrong with aluminum at these large sizes, but you do need to use special splice techniques that are safe for it. Look closely at the writing on the other wires. If they say "Copper" or "Cu" that settles it. If it says "Aluminum" or "AL" that also settles it. If you can't find any such markings, look end-on to the wires - you'll see copper in the middle of the strands if it's tinned copper. If not, it's aluminum.

If it is aluminum, look at your NEMA 14-50 recep labeling and instructions. Is it UL-Listed for aluminum? Then the wires can attach directly. If not, search for another 14-50 recep, or alternately, pigtail to some #8 copper. The best splice I know of for #6Al to #8Cu is the "MAC Block Connector". They're tough to search for thanks to Apple, but they're similar to large Alumiconns or small Polaris. Get your torques right and make use of the goop provided in the holes.