plugin ids should start at ID 1E6. The problem is, I am not quite sure that MAXON has never used any of these for their native tags, so you might filter out native stuff. To investigate that you could either check the tag types in the Python docs or, alternatively, grep the resource folder.
A less risky, but also less future proof approach would be to compile a list of all native tag IDs, again, the tag types in the Python docs and greping the resource folder would probably proof useful for that. I am not sure though, if the tag types list is complete (these id lists have a tendency to be a bit spotty). This would obviously have to updated once MAXON adds new tags, hence the lack of future proofness.