multiple default route base on physical interfaces

suppose if I want to completely segregate production & staging environment (separate physical interfaces, vlan, Ip addresses, GUI access, SSH access, Virtual Servers etc). In this case do I need to use route domain ?

route domain is to separate routing (not resource). there is still one gui. you may check about vcmp if it meets what you want.

Is it possible to use existing default route domain for production & create new for staging environment.

yes, you can do that.