There's only one downpoint of starting with a buddy, that is, buddys are a system for replacing transformations, so they would start with a buddy thus already using the ability, for that i think their powerlevel should start quite low, the lowest there is, however fusion would make up for it. Thats why i think they should be able to fuse at any time, but at their starting powerlevel, there would be 100% chance every arrow is random, as their powerlevel increases their chance would drop till when their CF bar is full then there would be 0% chance of a random. If it were possible to turn ssj as goten and trunks that would help when unfused as they might have to fight an ssj3 goku while fusion has its cool down because their powerlevel would be the lowest otherwise, a generic beam would probably kill them.
meh... just my thoughts