Is it OK to put a restricter on a new washer, because of old plumbing, and how do you do it?

Is this the common problem of the newer washers having an excessive discharge rate and overflowing the stand pipe? If so, the easiest thing might be to add a smaller section of hose or replace part of the existing hose. The back pressure from a smaller hose goes up sharply for small decreases in diameter. You could even add a partly closed valve. Ball valves are becoming easier to find, and give better throttling than the common gate valves.

I have worked in the chemical industry, and it is quite common to specify a centrifugal pump that should be larger than you need, and then throttle it down with a valve. Should work for the one in your washer.