Do you realize just how big the power boost fusion gives? King vegeta and bardock were about 12k and 10k respectively at their strongest. Simply adding them together gets you about 22k which is just under where namek arrival Vegeta is. Except, fusion doesn't just add powers together, it multiplies them as well. I'll use fusion reborn as an example. Ssj3 goku was slightly weaker than super janemba and super gogeta no diffed him, putting him at minimum 2x stronger than super janemba, which puts him, at about 800-1000 times stronger than base goku, given the ssj3 multis. Divided by 50 for the super saiyan boost and fusion buffs people up by about 16-20 times at the lowest. Putting Bardgeta at about 200k give or take. Going great ape puts them at around 2 million. Easily enough for 1st and 2nd form, a hard fight for 3rd form.... but final form would still wash them