the training sim seems to support that.
Of all the skills to train, one on one seems to be the best option for training players IMO. You can train driv/hand/js of relatively equal value or driv/hand/js/is with is/js half the driv/hand.
Also the sim seems to support that one on one guards trains bigs better than one on one forwards. Personally I'd suggest training bigs on one on one guards as it trains JS better than other options. Training one on one forwards is better for shorter players as training IS C is harder for them.