I have a 20-pound m250r2. I want to mount it somewhere, and want to know if a cheap Regent yardlight extention arm could hold it. I don't want the fixture to get damaged, so would it be strong enough? I don't want it to fall...

How thick is the metal, welds, base plate to which you mount it, and how are those mounted to the wall ?

If you mount it to a corner of the home, a plain straight 2 inch galvanised steel water pipe, mounted to 2 brackets on the wall (at ~2ft from each other), would do a perfect job and actually be more secure than any mounting that stands normal to the wall

Any extension arm will hold it as long as you secure the extension arm properly.  For example, if you mount it to a wooden shed, like I did, secure the mounting arm with a 2x4 behind the walls, and get some long and thick wood screws.  This will insure that the weight pulled will not yank the arm out of the shed.  Another suggestion, look for your local journeyman lineman around, and see if he has any mounting brackets that he has removed from service.  Most linemen will tell you know/they recycle/reuse them, but others will give you one if they don't plan to use it that day, as they can get one at their supply shop when they go and load the trucks.  The lineman around here gave me one that came down from the pole outside my house because he replaced it for one for a floodlight.  Then again, this was a few years ago before they started cracking down on inventory.  Now-a-days, at least around here, ANY working HPS fixture is generally re-used/recycled.  Saves the company (and in turn you, the consumer, money)