Zone Mortalis is a great way to play smaller games of 30k, and it also allows for more non-standard stuff as certain units don't work in ZM.
Calth and Prospero are great buys for any ZM force.
The MK3 marines can easily be turned into breachers with some boarding shields.
https://www.zinge.co.uk/products/siege-shield-x-5-sprue-human-scale
The terminators can be used as either generic terminators or as legion-specific terminators.
A simple way to do balanced forces would be to take a single legion-specific unit then build the rest of the army around it. For example, you could take some
IH Immortals alongside a breacher team.
Similarly, a Death Guard contemptor or some grave warden terminators alongside a tactical squad would be cool.