Hello Belabacsi,
I've found configuration examples that show that real ip addresses can be in a different IP subnet when using IOS SLB.
To be noted that when server load balancing is made on a CSM or ACE module used in bridged mode as we do, VIP ip addresses and real servers ip addresses have to be in the same subnet.
From the MSFC point of view the next hop to real server is the FWSM outside interface.
the default gateway for real servers is one FWSM internal interface.
I think it could work but I have no direct experience of this.
Hope to help
Giuseppe