The Zoeller 508-0007 ProPak98 does include all check valves needed, so you can remove the check valve that you currently have in place. Having more than the required or needed check valves, will add more friction loss which equates to reduced flow or GPM and could pose more issues down the road as the check valve ages and starts to leak or fail. The hole that you refer to is most likely a weep hole in your discharge pipe. The "weep" hole as it is called should always be drilled in your discharge pipe to prevent an air lock. This is true in sump and sewage applications. Air locking on a sump pump happens when there is air between the pump and the check valve of the discharge line. The trapped air prevents the impeller from drawing in enough fluid to open the check valve to allow discharge.
The weight of the fluid on the topside of the check valve keeps it closed. The problem is often noticed after the initial use when the pit has had time to dry out during a dry spell. When it gets water in the pit again, the air lock condition shows up. The pump runs due to the float switch being raised, but can't eject the water because of the air lock condition. Due to the design of the ProPak combo system and the placement of their check valves, this hole is not required.