Richardson is a very nice player. May develop into something down the road. The problem with him on this roster here in 0-12 to me is I'm not sure where he fits in schematically.
To me is very similar to Marcus Dixon, good size but not great. Probably is best as a 5-technique DE or a 3-technique tackle. The problem is, I don't know that he has the strength or the "anchor" to take on two offensive lineman which he'd likely do from both of those positions. He has a nice burst off the edge but he's not nearly fast or agile enough to play consistently on the outside.
Aside from all of that, who would he play in front of? Wilkerson? Coples? He might give you a little more pass rush than Mike Devito but Devito is about as stout against the run from his position as you can be. Those guys and a guy like Marcus Dixon at least have the strength to hold the point of attack. If he clears waivers, certainly a guy you snatch up for the Practice Squad.
As for Harrison. I think he is a MUCH better prospect right now. He does pretty much exactly what you want your nose tackle to do. But with as much versatility as this team has with the 3-4 ends being able to play tackle and your OLBs being able to play end in four-man fronts. There just isn't a need for a 3rd nose tackle that wouldn't even contribute on special teams. I think there is a very decent chance he clears waivers as I think he wouldn't necessarily be better than anyone else's second NT which he would need to be if you were going to take him on your 53.