One potential issue. You can only have one DHCP server per network. You may also want to consider putting this new environment on another physical or virtual lan or not using DHCP for the is AD environment.
You could create a new scope specifically for this new domain.
You'd still get issues if both scopes are on the same physical broadcast domain (switches), when a request gets sent out the quickest server to respond offers the address. Better bet for a test environment would be to use static IPs so that there is no confusion and no need for a scope.
Better bet is to build it virtually, if you've 8GB ram on your machine should be doable using vmware workstation then it's completely isolated.