Installing modern Deraileurs in an Old type mountain bike Frame
Yes and yes. The "everybody else" hanger adapter does indeed fit all screw mounted derailleurs except Huret. The size, thread and location of the mounting hold are all copied from the first Campagnolo derailleurs. The only complication are direct mount derailleurs, which usually come with adapter for frames with standard mount and can be used with the claw mount adapter too.

Of course, you can still buy a modern-ish entry level derailleur with claw mount.