stuminnis

I'm working on my first refurbish job, turning an old Schwinn Worldsport 14 speed into a 7 speed town bike for my 14 yo daughter. Most of it is pretty straightforward except that I need to change the shifter for the rear derailleur to either a twist or thumb shifter that will work with city bars instead of the drops currently on it. The label is gone from the read derailleur, but I have no reason to think it's not the original, and the current shifter is a stem-mounted index road shifter marked "Shimano 7-speed SIS."

So my question is, how specific do I need to be about compatibility on this? Will any 7-speed twist or thumb index shifter claiming Shimano compatibility work here? Or is it more specific than that? (Please don't tell me I have to replace the derailleur. I'm trying to keep this on the cheap.)

Thanks in advance.