My understanding (maybe I'm wrong here?) was that systems which do not store, process or transmit CUI data can be considered out of scope with regards to NIST 800-171 and CMMC compliance.
On this basis the development environment systems would only need to be included within scope if actual CUI data touches these systems in some way. If only junk/test data is used on the development systems and there is effective network segmentation implemented between the two networks then I would have thought that would satisfy the requirements and auditors with regard to technical controls. Whilst the physical cabling maybe connected through a managed switch you will likely need a firewall somewhere though that manages and restricts data flows between the two network segments.
In a production database if records containing CUI are marked as such by having a 1 in the 'cui' column for example, then when extracting a recent set of data to use to aid development and testing, anything with 'cui'=1 can easily be excluded.
To address the people and process aspects of limiting your scope, your policy could explicitly state that it is prohibited for CUI to be processed, stored, or transmitted to or from the software development environment, and your developers could be trained in what CUI is and where it is and isn't allowed and why. Signed attendance sheets at this training could be used as evidence to auditors.
I'm by no means an expert on this and am also having to figure all this out at the moment but to me the above seems like a reasonable approach.
