The slinkers really are evil (our point of view) / determined not to be crushed by human "monsters" (their point of view), and achieve steam engine tech levels while looking for an answer.
So they start to build giant (by slinker standards) steam-powered mecha to drive the humans off Io. Early models are about the size of a small dog, but soon they will be much more of a threat...
Slinkers use the automaton construction rules from FF IX, and slinker pilots instead of Babbage engines, starting off with a budget of £50 per slinker village for the first round of construction... then £75 after 1D6 Io days, £100 after another 1D6 days, and so forth. Their mission is to crush the humans or at least drive them off Io.
Humans use the tech they've got - by the time anyone can get help from anywhere other than the Jovian moons the war will be over. Their mission is to survive / wipe out the slinkers / evacuate as many refugees as possible, whatever seems appropriate.
Still seems to make sense now I'm actually awake. Any suggestions?