I'm looking specifically to run a Mech/Combined Arms Sci-Fi campaign using Solar System.  I recently purchased some Mechwarrior Hero Clix and am looking to use these in the game.  My goal was to create a flexible and abstract large scale combat system for Solar System where I could use these miniatures and all I could keep coming back to was Burning Empire's Firefight! mechanic which possesses the perfect level of abstraction.

So I began to convert Firefight! into Solar System rules and found the task to be not too difficult.  I need to do a lot of play-testing but before I can get there I need to resolve a couple of problems.

To give you a high level run-down, you basically set stakes for the conflict, mark out key pieces of terrain, and then perform specific maneuvers every round.  Maneuvers include things like "Advance", "Direct Fire", "Suppressive Fire".  Each side has a Disposition which measures their health (similar to Harm).  These maneuvers will function like normal Solar System Extended Action rounds where they will either be opposed or parallel.  Normal damage from these maneuvers will be applied against your units disposition - rather than affect individual characters in the unit.  A unit may consist of a single person or a squad of 10 men.  I hope this is a satisfactory basic description.

Here's where my conundrum is: If you choose Direct Fire you are able to take individual firing actions with one or more characters in the unit.  Your attacks affect individual characters in the enemy unit you are targeting as well.  This is all fine except for the fact what happens when a Mech or Tank targets an infantry squad with Direct Fire?  How can I possibly apply Harm to individual characters from vehicular weaponry? 

I understand Solar System does not concern itself with this level of detail but I was really jazzed about my conversion until I got myself stuck with Vehicular combat.  I suppose the easiest solution is to hand-wave it: "Ok, you did 3 Harm in your mech, you see a large swathe of infantry cut down before you."

Also, I am open to other suggestions - even a recommendation of another system/idea for using Mech miniatures in a Story Game.

This is an interesting topic. I'm not sure if I'd go for the Firefight rules myself, but that's just because I've been figuring out other ways of doing mass combat.

A simple answer to the issue of how to apply Harm to multiple characters from artillery fire or whatnot is to have all targets suffer the same degree of Harm. The SS booklet discusses this: when your attack is such that multiple characters need to answer it in extended conflict, the opponents all answer it separately and suffer whatever amount of Harm you're causing. Effectively you multiply your attack against all targets it affects.

The above of course assumes that you have multiple characters with actual dramatic intent in the scene and in the conflict. My basic principle is that characters who are there as tools for other characters do not suffer Harm - if they're extras, they die as simple color, and if they're real characters, they have to be drawn into the conflict as real participants before their injury is up for grabs. The new TSoY book handles this sort of stuff in detail by making units of infantry into equipment for their leader: the leader makes the ability checks and such, and the unit as whole can be broken just like equipment, but individual members of the unit live or die as dramatic sensibilities dictate.

In this approach you wouldn't have a specific number of men in an unit; rather, you'd have a "Great (3) unit of infantry" or some such, and then when the opponent attacks the unit, their check result is compared with the quality rating of your "equipment": if they beat the rating, the unit loses coherence and its fighting ability (which probably results in bad losses), making it useless for the character until and unless he can fix it later. The issue of applying Harm to multiple characters never comes up, because each attack is really applied either as Harm for the commander of the unit or as an attempt at breaking the unit itself.