Can I use a Shimano RD-M735 rear derailleur with road shifters like the SL-6400 (7s) or the SL-6401 (8s)?
Yes, it will work perfectly. Every pre-10-speed Shimano MTB rear derailleur will index with every road shifter prior (chronologically) to the beginning of 11-speed road, except for 7400/7410 Dura Ace.