My wife and I both own vintage bikes, I've had lots of "practice" on the mechanincs. I'm doing well. My question is this, if her bike (80's Nishiki Landau) has a 6 speed freewheel could I use a rear derailleur that were say 5 or even 9 gears and just adjust the limit screws? And how do I know what will or won't work with friction shifters?

Thanks,
Joe


Edit: Bike is full Suntour. Originally had an ARx but that was replaced with an older Honour. She drops her bike a lot.