Going SSJ3, Goku explained that when he was showing Trunks and Goten he said "First go SSJ then take it up to the next level. This next power has to come from deep inside..." and then he does it. Now I supposed the reason why Trunks and Goten could go SSJ3 is because when they fused I guess the human blood in them canceled out and the saiyan blood stayed so they were a full blooded saiyan. At any rate. A mystic can't go SSJ. And yeah Gohan cheated 2 times with the power up thing. Guru unlocked some of his power on Namek and then the old Kai unlocked the rest of it. His power if I remember correctly was in the Billions when he went Mystic. That places him somewhere between SSJ3 and SSJ4. Stronger than 3 but weaker than 4. If we were going to stick to the show as much as possible. Meaning only DBZ not DBGT (which wasn't even Akira Toriyama). Then there would only be 3 transformations per character. I guess I could list them out.
Goku - SSJ, SSJ2, SSJ3
Vegeta - SSJ, SSJ2, Majin
Gohan - SSJ(kid), SSJ2(kid), Mystic(adult)
Krillin - No clue how this works
Piccollo - Could start with training stuff, Fused with Nail, Kamiccollo
Frieza - 2nd form, 3rd form, 4th form
Buu - Skinny(PINK!), after absorbing others, kid
My method with Gohan is because he was stronger in SSJ2 as a kid than as adult. But when he went Mystic he was the strongest he could get. And the others are all just logical. Especially how Buu's grey form was weaker than the fat one that was together.