Yeah, a longer time to build rage would be better, with the exception of saxton hale since, y'know, he's basically the default hale. But maybe actually the disadvantages you said could be used for all the other bosses.
CBS- longer rage buildup
Vagineer- 15% Less health
HHH- Slower hit speed
Easter bunny- Slower hit speed
Either that, or it can just randomize, and doesn't matter what hale you choose.
Also, I forgot to mention, being able to choose Hales would let those get better experience on each hale due to low probability of getting picked to be a special one. Also, I don't think it's an advantage for experienced Hales, because I've seen CBS get pounded or the HHH because of their disadvantages.