I can understand Kate's feelings and ill-will about being upstaged by a young sexy newcomer. That's normal.
But that's when professionalism comes into play, in any line of work.
I could understand if, for example, Garrett acted up in such a poor fashion. But Kate is the experienced veteran performer, the "leader" of the cast and show. I'd expect her to set a better example and behave more professionally.
Plus, so many scenes and eps with Seven were directly about her interactions with Janeway. More scenes for Jeri usually meant more scenes for Kate, so what's the big deal?
Actors and actresses are people too and sometimes things are taken personally. It happens even in office jobs. It happens at my own work and we aren't in front of millions of viewers.
Yes, everyone should be professional.
But sometimes things just hurt and you can't really hide it.
Having seen Kate's personality through interviews, I highly doubt Kate was causing a ruckus and yelling at Jeri Ryan...I don't think it was that level of unprofessional. It was probably just a general animosity and also having to go to the set day after day with the hurtful feeling that she might not be "good enough" or something, compared to Jeri. Which I don't think is true at all, but in general many actresses are insecure.
Being upstaged is never a good feeling for anyone, actress or not.