I''ve seen Solasferas in person and they are cut in a way that is noticably different from a RB (which is the whole point). Other than the fact that they look "different", I don''t see a reason why you can''t mix the stones...however I can''t find a compelling reason to do it either.
As JD mentioned, I see no reason why you can''t mix the cuts though I can''t figure out why you would want to do that~? Small side stones aren''t that expensive even for branded cut diamonds, unless you were thinking of something big like 3ct center and 0.6 ct + side stones or something~
The cutters of solasfera or eighternity should have melee stones for their brand of cuts stashed away somewhere I am sure~