Pretty sure it's 41* and up, could be wrong...

3 people with diff info. yall prob can't even do gr40+ hence don't have the correct data NOOBS

The best way is to do whatever is fastest if it's quicker to do normal rifts on t6 do them.

Alterntatively do grifts at a slightly lower level to what your max is preferably around level 30 to increase your clear speed the thing with grifts is most of the time your garaunteed legendaries it's possible to go through normal rifts an no legendaries pop.

Don't forget keep spending shards concentrate on putting together a decent set together e.g Natalyas for the demon hunter as this will help push you further just from the set bonuses alone.