This is how I play Warhammer 40k with my girlfriend*:
-Go to the store and buy whatever minis you like, ignoring what faction, system, or even game they belong to. Cool-looking-if-slightly-out-of-scale plastic dinosaurs count as "minis".
-Organize the minis into "units". A "unit" is defined as: "any group of soldiers that always fights together"--a unit can have from 1-5 members. That is: one big demon can be a unit, three witches can be a unit, five space marines, whatever.
-Stat up each unit. Use any system you want. (We use modified 1st E. Warhammer 40k stats--ballistic skill, toughness, wounds, etc.--but without the morale rules) Here's the trick: each unit must be, theoretically, exactly as useful as each other unit. That is: the five space marines, as a group, must be worth as much as the one demon by itself, or the three witches. Weak units can be easily made more useful by giving them a good movement stat, good weapon range, or some sort of magic weapon or spell. Each mini should do roughly what it looks like it does, but it doesn't have to do exactly what the manufacturer intended it to do. If a scatter laser looks like a gatling gun, well, there you go. Put each unit's stats on a different piece of paper.
(-If you've never played a wargame, write your own rules or modify the stats from an RPG--just remember:
+Minis should have a "movement" stat, in inches, so you can move them around.
+Weapons need ranges--in inches.
+An average mini getting (succesfully) hit with an average weapon should die instantly rather than lose a certain number of hit points or wounds--otherwise you'll be there all day. Only an extra powerful mini should be able to survive more than one hit and only an extra powerful weapon should be able to cause more than one "hit" worth of damage. In academic terms, it's like this: in an RPG, weapons are things which wear away your hit points at a given rate--big weapons do it fast, small weapons do it slow--whereas in a wargame, weapons are things which have a given chance to kill an ordinary mini outright--big weapons have a big chance, small ones have a small chance.
+When in doubt: rulings, not rules. Play with someone who is not a dick.)
-Lay out terrain. Complicated terrain with lots of cover is best--books and cooking utensils work well.
-Place the units on the terrain, anybody can put any unit wherever they want, but units with more than one member have to stick together. (We are not dickish about unit coherency, but 4-5" from each other is best).
-Randomly distribute the unit stat sheets between all the players--whichever stat sheets you get are the units on your team, and wherever they are on the terrain is where that unit starts. (Alternately, you could use the schoolyard-draft sytem where you take turns picking which units you want.)
-If you get a unit you don't like and another player thinks that unit is viable, you can trade units with that player.
-Once trading--if any--is done, start playing.
-After the game, talk about if any minis conspicuously underperformed and decide whether it was the minis' fault or the player's fault--if it was the minis' fault, stat that unit up with slightly more power for next time.
________________________ (*Just googled the phrase "warhammer 40k with my girlfriend"--one result.