Well, if James' one finger worked, why not? Actually, I find myself using my index finger alone quite a bit, and I've been doing it for 45 years. But I do use index and middle for faster licks, too.
As for learning his licks, you can buy the "Standing in The Shadows of Motown" book, but you'll need to be able to read music. Same for "Motown Bass Classics," which also has tab. If you have good ear, you may be able to pick up a lot from listening to Motown recordings -- that's how I and many others figured 'em out. Technique -- probably two fingers, but there are pick players who can nail James's technique.
But what made James special was his ear -- the ability to come up with non-standard bass lines that complimented the melody and, while sometimes "busy," but didn't get in the way. That's something that is not easily taught. It helps to know chord structure and to have a good ear for what fits the music. You certainly don't need to limit your lines to roots & fifths only.
It may also help to take lessons from a bassist -- not a guitar guy who teaches bass. Almost any professional bass player -- certainly anyone older than 40 -- has been influenced by James.