The ETR insert will not work. You need to get an SQ 120 insert. 120 can be used in a 220 insert, but might have excessive tension. I think it's fine, but you'll have to try it to be sure.
220 in a 120 back may not be held tightly against the film plane, so is contraindicated. With 120 in a 220 back there's no such problem, so long as the plate moves far enough back to accommodate the increased thickness.