It isn't perfect, but it can help you to get a feel for how different shapes affect the tension and compression loads on elements of a truss.
A couple of rules clarifications:
The bridge must fit entirely within a 50 x 20 x 10 cm rectangular volume. This implies nothing about how the bridge can be oriented with respect to the support platform, aside from the obvious constraint that the bridge must span the 30 cm gap between the support platforms. So if you want a bridge that is 20 cm wide and only 10 cm tall, that fits the requirements of the rules.
While the load plate is flat and 5 x 10 cm, you don't necessarily have to have a flat spot in the bridge deck that size. If you have a curved deck surface or whatever such that only a fraction of the plate contacts the bridge, that's fine. It just needs to have some part of the load plate applying the load to the bridge deck, and only the bridge deck. The deck itself must be supporting the plate, you cannot, for example, have the plate sitting on the bridge side beams in addition to the deck (beams under the deck are fine of course). Essentially, the load plate has to go where the cars would drive.