Hi,
SBG will not interfere with the downstream email server and should work OK. It runs independently as a separate host that will relay inbound messages to your email server and accept outbound messages from it if you configure SBG to do so.
As for mdaemon there's not a Symantec Mail Security (SMS) version for that product. We have versions for Exchange and Domino and other solutions that may suit you.
The following link contains a list of our product portfolio:
www.symantec.com/business/products/allproducts.jsp
Regards,
Federico