Ed,
Hickory won't NEED a backing as long as the grain is straight, but I don't rawhide can hurt, thats for sure.
My thought on ERC are this:
It is a light wood, so it needs to be wider than any hardwood. To be safe for a 50# bow, I'd say 2" wide minimum (2.375" wide for 60#), from fades to midlimb or 2/3 of limb, then taper to 1/2" nocks, and 66"-68" long, with a 4" stiff handle, 2" fades. Thats for a 28" draw. Add 2" for each inch of draw past that.
I'll also warn you that even at 2" wide, it's not overbuilt and you'll need to tiller well. If you want to be safer go 2.25-2.5" wide.
ERC is light weight so even at that width it's not going to be a really heavy limb or anything.
ERC can also handle some fairly extreme abuses like heavy sinew backing and short recurve designs, but they also must be fairly wide, and really take alot of care and a bit of experience. I would not recommend something like that until you've got a few more under your belt.
On the plus side with ERC it works realy easily with a scraper and hand tools so hopefully it won't bother your joints too much ... I know mine appreciate the easy work!
Hickory .... I love hickory, I think you can;t go wrong there. 50-60# shouldn;t be a problem with hickory either. I'd probably go 1.75" wide for 50 and maybe 2" wide for 60# at 66" long also for a 28" draw .. same deal as before.