I think when you create your boxer, you get a certain amount of points to spend and everyone gets the same. The only thing is that perhaps Legacy Boxers could get a much higher rating with all the training.
It isn't hard to put a rating cap on boxers for online matches to promote fairness. 90 can be the cap. Seems fair.
My created boxer (non-legacy) is rated an 87. I think Houtz's was rated the same, can't recall.
And to fight in multiple divisions, you would have to create a boxer for each respective division. Unless you went and edited division and weight every time you wanted to fight in a diff division.
Although, iirc, there is a way to duplicate your created boxer. for example, I could duplicate my heavyweight 6'0" 220 boxer twice and just change division/weight for the 2 duplicates. Everything would remain the same including style/accessories/face but the weight would obviously be changed to mirror the division I'd want him to fight in.