There has been a lot of discussion on Facebook about these
There is a large difference in velocity of mixing 121 and normal paintballs, whereas with first strike there is a reasonable difference
So with first strike you can mix as long as you set the velocity for the fastest (fs) shooting normal paintballs at a lower velocity, 121s will have to be fired alone
People are unfamiliar with 121s and there has been some criticism of the design, and for the moment it looks like many events won't be allowing them