Any 7-speed Shimano cassette (or freewheel, if that's what it uses) will work. There is no need to match the group.
The RSX 7-speed will work fine. Are you replacing other brifters or downtube shifters?
Yes, I would like to convert my Paramount to Sora Brifters (' has downtube shifter) and I want to do the same with my Raleigh R300. The latter has an RSX group (low end brifters, I know ), with the left brifter failing from time to time (down shift tab won't catch). I thought I'd just replace both brifters on the Raleigh just to keep it uniform, but I want to make sure that the FD, RD and rear clusters are compatible with Sora brifters. If this isn't the case, I'll just go with a set of barend shifter. I'm really hoping it would be compatible as I like brifters better.