Cassettes on different wheel sets
 

Could I use different cassettes (11-28 and 12-25) on different wheels without having to adjust derauller?

Yeppers..

Yes, the cassettes are virtually the same.

Sometimes you may need a little spacer on the hub, but as long as you know which wheels need spacers and which don't (something you should know if you're swapping cassettes) you'll be fine.

If you size your chain for the (11-28), then using a wheel with the (12-25) mounted on should not be an issue. I have a total of seven wheelsets, sized my chain to my Ultegra (12-27) cassette, and all is hunky-dory.

your question is confusing. are you simply switching cassettes from one bike to another or are you also switching the cassettes and wheels from one bike to another?

+1.

+1

Good point. If you are switching wheels you might have to adjust the brake pad position so it will work across all wheels, except for may be opening or closing it a bit if the rims are wider or narrower.

I need spacers, but haven't found any thin enough. anyone have sources ?

If you have different brands of hubs, then the spacing might be different, requiring spacers, or derailleur adjustmnet when swapping wheels.

yes you can!

shouldn't be any issue, worst case you need to reindex the RD which takes all of 30seconds.