I don't know what the specific test requirements are, but the rpm readings for my 1987 300E were about 1700 rpm for both 15 and 25 mph. You may want to call a couple of test-only places (maybe even the shops you went to) and ask them how this works - how their machines are calibrated, etc.

If you think the first shop was pulling a fast one, you should contact the Bureau of Automotive Repair (http://www.smogcheck.ca.gov)to file a complaint. They get a lot of complaints about the whole smog check deal (surprise, surprise) - I bet they don't appreciate getting extra ones because some shops are being sleazy.

-anthony