Can I have a closed MEP inside an open MEP?

I have a plan that was a controlled group/ASG in the same plan.  

This year (2021) four other companies joined the plan that aren't part of the CG/ASG, and no other business ties, so we have an open MEP.

As I understand it, in an open MEP, each plan has it's own 5500, and in a closed MEP, they only file one.

So, in this case, can I treat that CG as one closed MEP and file a single 5500 for those plans, and stand alone 5500's for the four new ones?

Or do I have to file 14 seperate ones?

The original entities would still be a single employer for plan purposes. Just because unrelated employers are added doesn't change the original entities from a single employer to a closed MEP. 

Sounds like you originally had a single employer plan. and now you have a single employer(comprised of the original entities) plus a bunch of new entities that make it an open MEP. 

So that single employer can still file only 1 5500 and I will only have to do 5 filings.