They're a close quarter combat (CQC) unit, so they need the smoke, like Shocks.
However I imagine them to be either solely an AI unit (completely outfitted with Thomsons) or have a shorter range "sticky grenade" for use as an AT snare, similar to the one in CoH, or the satchel is already that? I don't really know.
I don't think they need smoke to be a CQC unit: Rangers and Thompson Paras don't and they're amongst the best. However, I do think the smoke's good for their role: they're vehicle support infantry and the ability to blind AT guns definitely helps with that.
I can see the difficulty in designing Cav Rifles: make them too weak a CQC unit and they don't get used, make them too good and they'll go off to the flanks to wipe squads instead of supporting vehicles.
What if they removed Combined Arms as a Commander ability and made it a passive on the Cavalry Riflemen? Have them about Assault Engineer effective on their own, but they get buffed up to Thompson Paratrooper performance with a vehicle nearby?
That gives you very efficient CQC infantry but only when fighting alongside a vehicle.
You also keep the highly thematic Combined Arms effect but escape the myriad balance problems of having it apply to anything.