@Randall used his sugar cookie pentagram to summon me so I guess I better respond.
Some of the issue here is that unless we made an army for them we have no way to properly gate model access in the app for cross faction play in Unlimited. It is part of why Exulon Thexus can attach Warjacks when he shouldn’t be able to in Unlimited for example. I could make new armies in Unlimited that are more restrictive but that didn’t seem very … Unlimited?
When we were picking Merc armies to make for Prime Arena we wanted ones with as many Merc choices as possible. To get the most bang for our buck and bring in the most Merc models into Prime Arena as we could. It would be pretty weird if one of the Merc armies in prime was composed of other faction’s models. Balancing models over different factions and in different armies is also complicated, usually reserved for Casters. Rahera for example.
Could something app side or prime side change that enables us to bring in these options into the game some how? Maybe, but …
If your playing Unlimited with your friends I would encourage you to bring what ever sparks joy and have fun playing models together. Don’t let a stuffy app tell you what you can and can’t do 