Pitching yeast at 50 should be fine: it'll be a slow start, but they'll wake up as the temperature rises. I'd definitely try to get the yeast in there sooner rather than later: the longer you leave you wort sitting around before pitching, the more chance for unwanted visitors to move in while the yeast are absent.
What I wouldn't do, though, is pour 60 degree yeast directly into 50 degree wort. A sudden temperature change will shock them. I'd put your yeast vial back in the fridge for 20 minutes to let it cool more gradually, then pitch once it is the same temperature as the wort.