This is more of a planning question rather than setup perse but this was the closest forum I could find. I would like to know if there's any MS published document stating which roles should/shouldn't be installed togheter on servers. Our current servers where already installed when I arrived and to name an example our DC and BDC one runs Exchange and the other runs SQLServer. which I believe is a big no no. We'll be updating our hardware and would like to avoid this mistakes from the get go if possible. Thanks.

Hi,As recommended by microsoft.Place the RID and PDC emulator roles on the same domain controller. Infrastructure master should be located on a nonglobal catalog server that has a direct connection object to some global catalog in the forest, preferably in the same Active Directory site. At the forest level, the schema master and domain naming master roles should be placed on the same domain controller as they are rarely used and should be tightly controlled. Refer the below article for more clear information.http://support.microsoft.com/kb/223346

Great, that helps. How about the roles for other servers? like DHCP, DNS, Exchange etc.. Is there anywhere anything written about which roles should/could be combined and which shouldn't?

It depends upon the plan of design. DHCP Concepts doesnt have any think like replication.Dns can be integrated with ADS and replicatedamong, roles can be placed.again it depends on the design in the infrastructure.