What happened to the original 6?
I vote for the above mentioned breeds. If you had a bit of a pecking problem, try getting one less hen. MOST of the pecking order problems is caused by not enough space.
Flock numbers are not a static, this is a perfect count thing. It depends on the age and SIZE of the bird in the set up. Often times you can cheat a bit when the birds are young, or in summer, but come the fall, you need to be ruthless and go into the dark days of winter, on target of slightly below the target count. The nights are long, and they spend a great deal of time roosted up.
As for longevity of hens, really you cannot reliably expect them to lay forever, it works best if you have a multi-generational flock, which might be difficult with just 5, but realistically, you need to remove 2 and add 3 kind of sort of, each year,(this allows for the unforeseen loss of a hen). That will keep your flock into fairly stable egg laying years.
Think of having the flock as a constant, but the hens in the flock may come and go.
Mrs K