Core / Edge is definitely the most versatile of the topology choices. It allows for easy scaling.
Assuming you have HP – branded Brocade switches, consult their design guide:
Or you can find Brocade’s design guide on brocadeconnect.com
Now, to merging the fabrics. I am assuming you have a redundant fabric right now. The new switches should be assigned unique domain IDs. Make sure also that the existing two switches have unique IDs. The switch will have to be disabled to change them, so plan for that if you need to change one.
Make sure all zones and zone configs are unique. Telnet to the new switches and run cfgclear and cfgsave to “erase” any zoning configurations. From there, you should be able to connect your ISLs and then start configuring the fabric with the new switches. The existing zoning should remain intact using this method. Once the fabric is merged, you can browse to either switch and “see” the other in the column on the left.