There should be no issue with them being in the domain. The SEG and DS servers usually sit in the DMZ and most best practices say your servers in the DMZ should not be domain joined. But it is not going to limit you in any way.
I echo Luke's reply and had my SEG/DS joined to the domain while they were on the DMZ network.
Just make sure you don't have both SEG and DS on the same server.
I can confirm you'll have no issue joining your DS and SEG to a domain. This domain can be the same for all your WS1 servers, or a DMZ dedicated one.