it used to be easy to find this document on McData's site but now that it's all brocade.com it doesn't seem as easy to find.

In the absence of a more useful and complete answer make sure you check the interop mode and domain id of the switches. The interop modes have to match on all switches in a fabric and the domain ids have to be unique.

The good news is that the mcdata switches are very good about not merging fabrics if any pre-checks fail. If it does fail then the switch will likely show "invalid attachment" and have an additional message that will give you a clue as to why it failed.

I'd make sure the switch is set to never principle as well as making sure the interop modes are the same and making sure that the zoning information is blank on the new switch so it won't try to push any zoning data to the rest of the switches.