If you only have one SMP, then yes you could install the SMA and have it point to the live SMP; however, this would then cause that vanilla machine to become managed by that SMP before it has been imaged, which means that you would then have to delete its computer resource from the database and then delete its MachineGuid from the registry (I believe the new versions of DS do this as part of their prepare for imaging process). Using this method also means that you could end up with solution plugins also being installed, some of which may write the MachineGuid to their own registry areas which may or may not be targeted by the DS process, so you should manually check this afterwards before image deployment.
If you have more than one SMP, then building a SMP only VM to act as your redirection SMP allows you to still only have one image. This SMP also becomes usefull if you experience duplicate guid issues, as it can be used as part of your rectifying process.
Concerning your current setup, I assume you first import from AD, and then use the SMA's scheduled push feature to target the newly discovered machines, yes? If this is the case, are the newly imported machines found in the Target associated with the push? Also, how frequently is the push scheduled to run (by default its daily)?