I wouldn't worry about keeping the ring finger planted, I think you'll be fine as long as it doesn't get in the way.
From my experience, and this is mostly from guitar, but if you start to pick up the tempo and notice that your technique begins to change in some way, thats usually a bad sign meaning you're starting to tense up. The best way to build up speed is to just keep it loose and play slow and accurate. Its not really what your mind or your heart always "wants" to do, but building speed is slow gradual process, and as you're getting faster is best if you don't really even notice it.
Just play the same songs over and over again, use a metronome as much as possible, and never put the speed any higher than what you can do comfortably, relaxed and 100% accurate.
I'd like to hear some opinions on why you would want to plant fingers at all. Lots of teachers are against it as it can inhibit your range of motion if you pick the strings in different places to get different tones, and probably other reasons I can't recall at the moment. For banjo it seems that most people recommend planting the pinky and/or the ring if possible, but I'm not really sure why.