ATC will give generally you any available altitude you request, unless it is in conflict with route requirements. I once did a flight in an RJ at 12,000 feet. Way low, a few questions were asked but engine anti-ice was MEL'ed and even thought it was midsummer I had to stay out of icing conditions. I also flew one in to maintenance hub about 300 miles with the gear pins installed. Max Extended Speed 250. We got no more than 220 out of it don't remember the altitude.