I really like the idea of pairing them together during the regular season. As I mentioned in another thread, a lineup of Smart, Jaylen, Hayward, Williams, and Williams could be really effective in short stints.
Regarding Grant Williams as a long term starter, I believe his lack of athleticism will hold him back. I've seen some comparisons to PJ Tucker. The problem is that Tucker is in a very unique situation. I could see Williams as a high end bench player who plays 25-30 minutes. Who knows though, maybe the Celtics will construct a a championship roster where he can start at the 4 spot.