i think rpm ends work best with associated ballstuds. I had the same problem trying to use losi ballstuds and tried to work them around a little to get them to free up and wound messing them up. Basicly it didnt work, you are probably better off with a little slop than a binding turnbuckle until you can get the right ballstuds.
I'd much rather have them a little tight than sloppy. I went exclusively to RPM ball cups on all my HPI cars to tighten up the steering. Use a little powdered graphite as a lubricant. Works great, won't attract dust like a liquid lubricant will. Be careful not to scratch the balls as this will definately affect how smooth they operate.