This is the layout view. An airsoft paintball pistol of some sort (as yet yet undetermined) goes in the vacant slot in the right-hand side of the body, powered from the main batteries. A very small 12V battery powers the radio (to be concealed in the head). There is no elevate (so this is a short-range, or should I say single-range device). The legs will work, and in fact also steer.
The radio is fact a 3-channel garage door operator which is activated by a keyfob, the channels controlling left motor: forward, right motor, forward,and fire gun. That gives me steering but no reverse. It is very compact. Because the keyfob and receiver are coded, it will not be affected by other tanks/models. More specifically I can have a squad of (say) 4 robots operating together and each controlled by its own keyfob. I can also have one "master" keyfob which operates them all, so that they can move and fire independently or as a group.
There are a lot of issues to be solved there, but that's the basic idea. In terms of rules, the robots will of course be allowed to move and fire independently. They should be able to fire more than 1 shot- I'm not sure what my paintball pistol will manage but I should think we are talking about 4 or 5 each before reloading. They will need a lorry or similar to supply them and move them around the battlefield, as they are not going to move fast (or cope with rough ground very well).
In terms of effectivness, I suggest that 4 robot soldiers and a lorry probably take about as much effort to build as 1 tank or similar, so should have a similar level of effectiveness. To answer Neroc's point, an infantryman on a supply lorry is effectively the same as a lorry-mounted self-propelled gun. One thing I do want is for infantry to be something different from a tank. In real life, from a tanker's point of view a bazooka operator is a snake in the grass- almost impossible to hit when stationary (largely beacause you can't see him) but easy to see and hit when moving (this is what MGs are for). In our game we have no MGs, but my robots won't hide either (though they can take cover,and yes I am planning "Palmer's Portable Trench"- a sandbag redoubt for them to hide behind). From a tanker's point of view, fighting my infantry will be a matter of targeting 4 infantry before they shoot you from cover or charge out and get you at close range. Which should be fun, I think.
Well, that's my idea. Comments welcome!