The FedSuns seem to keep their units pretty homogeneous, and then putting those various homogeneous units together.
So a `Mech regiment is going to be all `Mech, and a Vehicle regiment is going to be all vehicles, but they might detach a battalion from each to form a temporary unit on a world.
That's part of the reason they created the LCTs...instead of having a RCT split up and covering several worlds, thus breaking it up into small pieces, you could have a single LCT on a world, thus keeping it together.