I've did lots of testing with both fixed and nonfixed bots before I released. Here are some things to note:
Bots arrn't solid. Collison detection just applies forces to their velocities. The forces are scaled by their mass so this can result in some strange behaviour. For example, if a massive non-fixed bot hits a lightweight non-fixed bot, things appear like you expect - they penetrate some, the massive bot gets deflected a little, the less massive one gets deflected a lot. But if the less massive one is fixed, the amount the massive one gets deflected remains the same so it will appear as if the massive one moves through the smaller fixed one and only gets deflected a little.
(Hmmm. This gives me an idea. Maybe I should treat fixed bots as extremely massive for the purposes of calculating the forces on the non-fixed collider. I'll try this...)
Also, bots under their own power can thrust against collison repulsion forces and penetrate. As above, small mass bots won't have a lot of inertia. It they thrust each cycle, they can overcome the collision forces. Additionally, ties connect to a bots center so for very large bots, the length of the tie "pulls" the bots towards each other and provides forces opposite to the colision force.
Multibots present some other strange problems because the collison forces can be translated to tension forces on the tie so tied couples can sometimes appear to go through large bots since the forces on the bots can balance and the ties themselves are not currently part of collision detection.
