Hi, about HI Boston Downtown, i should say that i didn't really have any problem with anybody there, in fact made new friends. I don't think there was any homeless or low life dude there but some people i met in the kitchen was not so young (about 40/50). The things i didn't like about the hostel was the price, the "not so willing to help staff" and the fact that you cannot drink alcohol in the hostel, taking into account that nightlife there ends at 2am which for me is really early. Everything else was alright, do not expect a big city full of life as New York, i've stayed in Boston for 3 nights but i could have stayed just 2. The city itself is small and you can walk to all the places worth of visiting. I hope this clarifies...
Hope you have an excellent trip :)