I am not sure about the early trigger theory with root restriction. I grew a bunch of stone dragons this winter and left them in 16oz cups for 21 days while waiting for sex to show and they stayed small yes, but I didn't see any early flower trigger signs from the root restriction. I figure root restriction started at 10 days, and it took another 11 days to show their sex.
The best thing you can do is research and buy strains that are meant to finish small. The many strains from the portal are very stout and would suit this application well. Good luck!