Bicycle Mechanics - What cassette for RSX 7 speed RD?

Actually 2 questions:
1. Will any 7-speed cassette/freewheel work with an RSX 7-speed derailleur? Assume I keep the gearing below 25T or so.
2. I noticed that virtually all wheels today are built with hubs for 8-10 speed cassettes. Any idea where I can find a wheel that would accomodate the 7-speed cassette?
Thanks for any help you can provide.

1) yes

2) assuming the hub will fit in between the dropouts on your bike, a 7 speed cassette can be used on an 8/9/10 speed freehub body. You will need a 4mm spacer to go between the cassette and the hub, and then adjust the limit screws on your rear derailer.

1) yes

2) assuming the hub will fit in between the dropouts on your bike, a 7 speed cassette can be used on an 8/9/10 speed freehub body. You will need a 4mm spacer to go between the cassette and the hub, and then adjust the limit screws on your rear derailer.
Thanks. Problem is I want to get a new wheel for use on the trainer with a Conti trainer tire, but having to adjust the RD each time I make the swap cuts into the convenience factor of doing this.

1) yes

2) assuming the hub will fit in between the dropouts on your bike, a 7 speed cassette can be used on an 8/9/10 speed freehub body. You will need a 4mm spacer to go between the cassette and the hub, and then adjust the limit screws on your rear derailer.

1. Agree with yes

2. If you use an 8/9/10-speed freehub, the 4 mm spacer can go either between the hub and the cassette or at the small end of the cassette between the smallest cog and the lock ring. The outside position may be easier since the rivets/bolts holding most cassettes together interfere with seating the spacer properly. Either remove the bolts or put the spacer to the outside.

Thanks. Problem is I want to get a new wheel for use on the trainer with a Conti trainer tire, but having to adjust the RD each time I make the swap cuts into the convenience factor of doing this.
You may need to do some digging, as 7 speed hubs are still out there. I think Nashbar still has a 700c/freewheel hub wheel in stock. You'll just need a 7-speed HG freewheel, which isn't too hard to find either.

Sounds as if you have a 7-speed shifter, too. The RSX rear derailler doesn't have "speeds" and is compatible with Shimano 6, 7, 8, 9 or 10-speed systems, as long as the number of speeds in your shifter match the number of sprockets (and in the case of 6-speed freewheels, there were a couple of different spacing variants back in the day).

The main culprit in your having to adjust the rear derailler each time you switch wheels is the spacing between the cassette and your bike's rear dropout. One of the cassettes sits further away from the dropout than the other.
So, you should
1) find the wheel where the cassette sits further away from the dropout (which is unnecessary and only increases the dish of the wheel, making it weaker)
2) remove a spacer from the drive-side axle and stick it over on the non-drive-side - if the difference is only 1mm and you have only a 2mm spacer on the drive-side, you may need to buy two 1mm spacers
3) re-dish the wheel since the rim will now be slightly too far toward the drive-side