Here are the Mice in Mech Suits mech combat rules; this is a public beta playtest, if you download these and use them, please give me feeback! These rules work much better for at least two on two combats, and I have ran them with up to 16 total combatants. This is a stand alone game, and it pretty much ignores the mech pilots (who are mutated mice from the Mutant Future!).
One cool thing about these rules is that the Mechs are roughly human sized, so they could easily be used as robotic adversaries for Mutant Future or any other edition of D&D, really. They are not so overpowered as to completely unbalance a normal game if they are introduced. For Mutant Future, small mutant animals or mutants with the Density Alteration, Self mutation could easily pilot them. The To Hit chart in the stand alone rules section is basically the to hit probabilities of a 5th level Mutant Future character, so you could adjust those numbers if you want the mech to be a more ferocious combatant.
In my campaign, the players have finally convinced King Arthur and the Mice of the Round Table to help in their cause, so a contingent of these mechs will join them if requested to sortie against the Knights of Genetic Purity or Julius Corple and his henchmen. This definitely gives the party some sorely needed muscle, because up till now, they have been running around like chickens with their heads cut off!