When i swapped my Shimano mtb triple for a Sram 42/32/22 I needed a shorter BB. On my old one the front derailer would not shift up to the big chainring.

The chain length should be fine going lower on the small chainring as suggested. You just don't go into for example 22 - 11.
I basically use the lower half of my cassette in the small chainring, all of my cassette in the middle chainring, and the top end of my cassette in the large chainring.