When I get my male cat neutered will my female cat be less aggressive?

Yes, most likely. Female cats are aggressive to male cats mostly during mating season, when they will be more guarded. But on some occasions, it doesn't work. I have a friend who has two cats, one male one female, and a similar situation. However, once she got the male neutered, they continued to fight. Since they range so much in age, and you've had Izzy longer than Jack, it's because Izzy isn't happy that another cat is in her territory. So I don't think neutering Jack will help, but it might. :) Hope I helped!