Though I am not a expert... what I can tell you is only my experience.... as far as installing a used ac... well it depends on the condition of that particular compressor... if is it still good and alive... i see no harm.
as far as you said Mehran is not fit for ac... well I do agree its not fair to but such a heavy burden on a 800 cc poor thing... and specially if it runs on CNG... but I have a 2004 Mehran... and till now there no problem with it both the car and the ac...all you have to is to be a little careful while driving in heavy traffic, you dont want your car to get very hot... And I guess they even have ac in a 660cc car.