Ive kept 2 pinkies and 2 sweep, all about 2 inches long, for about 6 months in a tank before releasing them....
Have you had the filter running in the tank for the past 8 months??? If not, i would suggest you run the filter for about a month before adding anything to it...test for ph, amonia, nitrates and nitrites...
Im not too sure how hard it is to keep trout, just make sure you do regular water changes...