With G1 Devs they could be all the same size, but for most of the other combiners, you would need a larger torso. They would have to include a cross-sell on the scouts and legends in the combiner so people actually KNOW what to buy to create the full combiner.
you realize that we have the knock-off versions of these G1 combiners in legend size already, right? They are about 6 inch or 9 inch tall (depending on which series you get them at)
I'd prefer Legends combiners that follow Legends Devvy with multiple bots making up the torso / head section opposed to PCC style bots on varying scales. G1 mini knock-offs have their place but these are not what people are talking about in this thread.
My idea is that they sell Legends figures separately that can manually be transformed into a limb for a PCC Commander. The spring gimmick gets in the way of a robot mode but remove it and have the limb be a simple one like in G1 -that should work.