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My club plays almost nothing but generic rule sets and there are quite a few out there that are really well done for free or very reasonably priced. A few my club has tried (and a few we haven't) that you should consider. Unless stated otherwise, these are free systems.

Warband Skirmish games (7-15 figs a person)

In The Emperor's Name (ITEN)
Based on the FUBAR game engine, this is a really fast playing game and alot of fun. It's based in the 40k universe with lots of scenarios and warbands and a really nice campaign system, but has a unit creation mechanic we use it for generic skirmishes.
http://thegamesshed.wordpress.com/2011/07/02/in-the-emperors-name/
ITEN is free but Osprey is bringing out a Steampunk version in print next spring called "In Her Magesty's Name"

Wastelands Meltdown.
Mad Max meets Fallout in a game. Really fast playing, but alot of fun. This game and ITEN both have good advancement and campaign systems.
http://s1.zetaboards.com/Post_Apoc_Wargames/topic/4047419/1/
Ran a convention game with this in the spring using a slightly streamlined (it's already pretty streamlined) version of the rules. Had a great time.
http://www.dakkadakka.com/dakkaforum/posts/list/447425.page#4237701

Blasters and Bulkheads
Based on the Supersystem game engine, this is a thinly veiled Star Wars game, but works for other sci-fi space opera games also. Has a unique mechanic that allows both heroes, and mobs of mooks/henchmen. Well worth the 10 bucks
http://www.wargamedownloads.com/item.php?item=983

Nuclear Renaissance.
I've not played it, but I've read it and it looks really well done. Sort of a Necromunda crossed with Gorkamorka Available as a print rulebook or a free download
http://www.ramshacklegames.co.uk/nuclear/index.html

Neutron York 3000
This is the game I'm most excited about. Really fast playing, with a good advancement mechanic and an interesting dice tally method. Very tongue-in-cheek style. The club is a bit post-apoced-out after our spring convention game, but we hope to try this one soon. Does not make much allowance for vehicles yet. 7 bucks
http://www.wargamedownloads.com/item.php?item=980&site=NY3Kblog

Platoon Skirmish games (20-40 figs per person maybe a few vehicles)

WarEngine.
One of my favorite games for sci-fi. Very versatile and fast playing. Only handles light vehicles easily, Heavy vehicles take some tweaking. A versatile Unit creation mechanic that does more than just give stats, it also incorporates flavorfull special rules. The yahoo group isn't active, but has all the files you'll need.
https://groups.yahoo.com/neo/groups/WarEngine/info

No Limits.
This is basically the open source version of the "VOR" game. Not played myself, but looks pretty cool. Alot of folks liked the VOR rules when it came out.
http://www.wargamesunlimited.net/nolimits/index.html

5150 Star Army
I mention this because while I didn't enjoy it, I know alot of folks that do. It's available in both a platoon combat ( Star Army) and individual, almost-RPG (New Beginnings) versions. It uses the Two Hour Wargames "reaction" system which takes alot of the control away from the gamer, but it can make for some interesting battles. 20 bucks
http://www.twohourwargames.com/scifi.html
If you're not sure about it, I recommend you try their free ruleset "Chain Reaction" which is virtually the same rules without as much Sci-Fi chrome. If you like CR you'll like 5150. If you don't, then steer clear of it.
http://www.twohourwargames.com/chainreaction3.html

Those are both good games, but neither is "Generic" meaning that neither has a mechanic for statting up figures of your choice for the game, instead giving you only specific army lists.

That said, if the OP has a ruleset that they really like, there's no reason not to tweak the rules to accommodate more figures. Gorkamorka in particular is an extremely easy ruleset to add to as it is based on the same rules engine as Necromunda and 2nd edition 40k. The points are different, but it's easy to port the statlines between 2nd ed, Gork and Necro with a few tweaks for balance.

I've read Mutants and Death Ray guns looks like good fun. The main difference (besides setting) between Flying Lead (FL) and MDRG is that FL has more extensive rules for ranged combat, with separate profiles for weapons. They both share the same activation mechanic, but in combat MDRG is much more similar to Song of Blades and Heroes, my favorite Fantasy warband skirmish game.

After playing a couple games, I don't like the FL firing rules all that much, but alot of folks do.