Originally Posted by
blamp28
You really need all of the drive train to be of the same speed. An 8 speed rear wheel will fit but not operate well unless you have 8 speed shifters etc. The 7 speed is most likely a freewheel but to be certain, either check with the LBS or got to the website of your bike's manufacturer and look up the specifications. Shimano and SRAM are both fine brands if you are looking for a cassette and are interchangable as are thier chains. I prefer SRAM myself but both work well. Just confirm what you need before buying.
Night before big ride and a mate of mine had a 7 spd cassette jump through wear. No 7 speeds in stock so fitted an 8spd cassette and left the 7 speed changers on. Just adjusted the stops so he could not get the 11t ring. Worked fine but did change the chain at the same time. Luckily he did have an 8spd freehub with a spacer for 7 in place so everything fitted and worked fine.
And 7 speed cassettes for freehubs are available still- you just have to hunt them down.