AltostratusAs Nikoolayy1 indicated, BIG-IP LTM does allow pool members in a single pool to be configured with different IP addresses and/or different ports. The only reason you would need an iRule (or local traffic policy) is if you wanted to conditionally select the pool member to load balance to based on some part of the payload or if you want to modify the payload for the server-side request or client-side response in some way. For example, if you wanted to conditionally select the pool or pool member to load balance to based on the HTTP host name or parts of the HTTP URI, you will need to use an iRule or local traffic policy to set up the appropriate conditions and actions.
You do not have to make any modifications to the HTTP host header unless the application server requires it. Otherwise, the host header will be passed unchanged from the client-side request to the server-side request by the BIG-IP system. In most cases, this is sufficient.
Other considerations, such as persistence, are not reflected in your original question, so it is up to you to determine whether or not you need it (or other profiles) on the virtual server and in what form. Certainly, if you want to modify the HTTP payload in any way - request or response - you will need an HTTP-type profile on the virtual server.
