The procedure linked by Vikas above shows how to implement this on AWS. The concept is that you can have multiple routing tables on independent virtual routers on the SRX. The fxp0 out of band interface is in the root or main routing instance just as you have it setup.
You create a virtual router and place the transit interfaces here and they get their own routing table and default route.
The alternative if you don't want to use routing instances is to point the default route out for the transit interfaces.
Then have only specific routes for the ip addresses you use to manage and connect to fxp0 pointed out the fxp0 interface instead of the default route.
this only works as long as those subnets do NOT need to use the connections via the main transit interfaces.
Steve Puluka BSEET - Juniper Ambassador IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP) http://puluka.com/home