It depends on what else you have running. Many applications, like Exchange, require a global catalog at each site. Even if you only have a single domain. Since it is only a single domain, there is no harm in making all of your DCs global
catalogs. It won't increase any replication, but it will make all of your DCs more effective because of those applications that query _gc. records.
Also, have you enabled the Active Directory Recycle Bin? Once that is enabled, it renders the IM basically useless.
So, only issue I see.... make all DCs GCs. Separating them is a early 2000's concept and no longer relevant.