So in matched all units have to have a matching faction keyword.
In open and narrative the same is only true of a detachment your army can include units of any factions.
An interesting wording on an astra millitarum stratagem take cover.
When an enemy selects a friendly unit as a target in the shooting phase. That unit gets +1 to saving throws until the end of the phase.
It specifically doesn't state astra millitarum you can use it on any unit, so take a small detachment for the ability to unlock AM stratagems and troll with a 3++ om your Stormsurge or get your biggest troll face out for a 2++ Riptide with nova reactor