Multi-LAN does not really play well with the shaper, currently. The only way (as far as I know) to handle multi-LAN properly would be to create a bridge with all the interfaces and apply the shaper to it. If you do that, although all LANs will be on the same subnet, you can still somewhat filter traffic between them (by activating the proper system tunables).
Anyway, bridging sounds exactly like you want. And "guaranteeing bandwith" makes me think of HFSC ;)