Antivirus on Exchange and DC

Hi,

I installed DC and Exchange servers and migrated 50 of 100 computers to this domain without antivirus on DC and Exchange. Is there any reason to install a new domain (with antivirus from beginning) and migrate again all computers ? or it's ok if i will install it now and scan both servers? The servers are behind the linux firewall and i have another server for edge (on linux)

Thanks.