if there are not enough pool members in a pool send the traffic to another pool

This could be performed via an iRule and having 2 pools. Pool A will high the 5 higher priority members and Pool B will have the 5 lower priority members.

when CLIENT_ACCEPTED {
     if { [active_members [LB::server pool]] < 4 }{
          Pool B
     }
     Else {
          Pool A
     }
}