You are right, most of the time the tyre shop will insist you re-balance the wheels and re-align also, so they can charge more. If the slightly worn front wheels are still in balance, transferring them to the rears won't be a problem and with the hardly worn rears, the balancing also should be intact unless some weight flew off. Do the balancing only if you feel they are unbalanced. Of course all these must be done with off wheel balancing and not on wheel. If always have balancing problem, probably need to look at fitting a center cone.
For alignment, just check the tyre wear, if they are wearing evenly, also do not need to do alignment every time.....
Well, for 2 tyre or 4 tyre change method, it is up to the individual. Both have pros and cons.
Just want to ask, if you are using different tyres up front and in the rears how do you do your rotation?