mostly I'll be trying to follow the setup in wolf and blake. which is roll 1D6, consult the chart for the mission, and that's what you're fighting. I'll probably be scaling it so that there's fewer mechs on the board - for example take the withdraw mission. roll a six. that's 6 PC mechs, enemies get three heavies and four assaults.
Scaled down, that's 4 PC mechs; 2 enemy heavies, and 2.7 assault, the fraction probably rounding to a medium. That's pretty much worst case scenario, since they start pretty much on top of the players.
where I'm willing to fudge is what of each class they face. 3050 grasshopper 'upgrade,' a good ostroc or ostsol, charger 3k, grand titan, ER-PPC clint. Fast enough to keep up with a lot of forces, fairly tough, but easy(ish) to outgun or survive while running.
Thanks for the ideas, BTW.