7 speed cassettes and freewheels are interchangable as the spacing is the same, even Campag will work (give or take assuming you are not looking at Regina Syncros or the like). 7 and 8 speed cassettes and freewheels are interchangable as they use spacing that is basically the same (slack will be taken up by float on the jockey wheel).
Big tip is to set the gears up with the chain on the middle cog of the cassette, not on the smallest or largest. That way you make the most use of the available float on the jockey - ie if the spacing is .2mm out on a 7 speed cassette on an 8 speed shifter and you set it up on the lowest cog then the difference will be .2x 6 = 1.2mm max when you hit the large cog, but if you set it up on the middle it will only ever be .2 x 3 = .6mm max at each end which can easily be taken up by float.
The only thing that you have to be careful with are the shifters and mech. If you use a 7 speed cassette then you could accidentally over shift unless you wind in the limiter screws on the mech.
Thanks very much for that man
It'll be XTR M900 rear mech and M900 front mech, a 7 speed Suntour or other quality Freewheel and Suntour XC Pro thumbies.