Adding MacOS partition to support both Mojave and Catalina

On my wife's work machine (self-employed), we need to be able to run Mojave (10.14.6) due to some legacy software required to support one of her clients. This has begun to create interoperability challenges with the rest of our machines and devices, so we'd like to create a new partition that would be current (for her, Catalina (MB Pro Late 2012)), while retaining an old, minimally-sized Mojave partition. (She is also running a Bootcamp partition for still other clients.) I note that her MacOS partition is APFS, so I *think* that this should be easy, but would appreciate guidance from the Smart Folks here about: what tools I should use to create the new partition; the sequence of steps I should follow; and thoughts about minimum practical size for the (soon to be old) Mojave partition. We have current backups (TimeMachine).

My *guess*: (1) create new Mojave partition under Mojave, minimal size; (2) load Mojave onto that partition, along with the legacy application; (3) upgrade the original Mojave OS to Catalina.

Thank you!