I think the real problem will come from mixing different armies in one foam tray.
If you went with pluck you'd choose whatever unit fit into the tray. But you might fit less.
If you went with precut foam you run into the problem of the odd man out syndrome but fit a few more in.
GW bags that I've seen have pre cut. My stores don't have much for them.
Sabol Army Transport has pre cut and pluck foam.
Battlefoam Army bags have per cut, pluck and custom foam.
So ask yourself what's available near you/what can you order? Also take that list and shop around, see how many of each kind (pre, pluck, custom) you'd need and what you can afford. This is, of course, manufactured army bags. DIY may allow for more customization but I don't do that. I'm lazy