I've collected second hand, 4 MTB style bikes over the past few years, and now actually sitting down and sorting out what to keep.

One I've stripped down as parts as it was quite old and had random parts added to it anyway....

So I'm now left with a Giant Upland SE, Mongoose Rockadile, and a Raleigh xxxx (TBC).

However one needs a new wheel and another one need a new cassette (21 speed) and looking online, I'm a little confused about the cassettes. 

Are these interchangeable across bike types, such as MTB, road, track etc ? So dosent matter what type I get long as its got 7 cogs ?

My daughter seems happy with the Mongoose so will leave her that and I'll prob use the Giant.