The shoot their resource as a mobile cannon idea is GOOD. But it have weird implications. What could you shoot at with it? Only fellow Tetsujin? Anything? If you shoot at a battlemine, with how much strength do they shoot back? or wouldn't them ever shoot back?
Perhaps it could be worked out with shots from Tetsujin being able to damage but not being able to "defeat" anyone. That is, you can take out a building or two (depending on what are you firing) but it isn't "combat", it's "opening fire" : it can only destroy, not defeat, vassalize, or anything.
As with the battlemine firing back, it would probably be best for them not to fire back, or fire back with a single cannon.
But of course, our beloved Rastie probably has all his ideas for the clanking steel men already set, so this is just wishful thinking.