Installing modern Deraileurs in an Old type mountain bike Frame Found my old mountainbike(steelframe) and i saw that its Rear Deraileur was worn out, was wondering would i be able to install a modern deraileur on a bike frame like this? I'd also like to ask if deraileur hangers shown in the picture with be compatible for this frame? Pls i badly need some advise about this as i want to restore this old bike 

Also! Can all deraileurs be fitted to ANY bike hangers or frames? (Do they all have standard bolt size and holes)
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.