For those who feel the Aliens were GIANTS, consider using 20mm to really make them look big. It sort of goes too far I think, but 20mm modern day plastics will do the job. Plastics = cheap, especially if you're just mucking about.
Other Scales
A company called Halcyon released a couple of Aliens kits which might be useful. Their APC is in scale with 54mm figures, the Leading Edge APC being lower in height than the figures it carries, and probably more suited to true 25mm. The APC just looks more roomy in the movie, and also when it runs over an alien when leaving the base it looks bigger. The Halcyon Dropship is 1/72, and therefore too small for our purposes. However, if you can find it cheaply (not on Ebay obviously) it can make a good base for you to do some scratchbuilding. For those not so fussy on using the movie APC, GZG make some resin 25mm APC's of varying types. These should do just fine. These are available through Eureka Miniatures, and the smallest APC is about $16. I've also got quite a number of 28mm walkers - Grendel, and AT-ST Star Wars walkers - which will do a good job as well. Also, don't forget your common everyday civilian vehicles, such as die cast dozers, trucks, etc. They make good scenery and cover, and won't cost you a fortune.
Macfarlanes have released some amazing Aliens in their Movie Magic 5 & 6 series. These guys are about 200mm high, and you get Aliens style warrior vs predator with 2 eggs and facehugger, dog warrior (Alien 3), Alien 4, Queen (Queen closeup) with webbed colonist, and one other I can't name right now. There might even be two alien vs predator sets. You might think 200mm is too big for wargaming, but let's not forget our GI Joe friends. Yes, you can hunt aliens through your lounge room and over grown backyard and use your GI Joe jeeps for a real purpose. Look honey, I am playing with the kids. No, honest! Slighty more expensive than buying 25mm figs, these guys are about AU$40 - AU$100 for the Queen. They are still available on Ebay. There are free GI Joe rules online, just try google search and modify to your Alien style.
Andrea are still selling a 54mm Ripley vs Alien diorama.
AEF released the whole cast of Aliens in 54mm resin in 1988. I've never actually seen any of the Alien warriors from the range up for sale, but the rest pop up on Ebay from time to time.
Also in 54mm, TreeHouse-Kids released a 54mm Aliens playset, containing plastic aliens and marines and a 2 level structure. Booster packs were also available, containing random mixes of alien and marines per box. The Alien figures aren't too bad at all, with 7 figures made up of 3 poses, 2 poses standing looking aggressive (pose 1, pose 2), and one lone climber. You also get 8 opened eggs (for some reason I thought you got facehuggers, but apparently not) a few crates, and then the two storey structure with steps. The figures are made out of a soft rubbery plasticy material, the marines in light olive green, the aliens in black, and the eggs in a black. The 6 marines seem quite thin, but not too bad. The Aliens suffer from pre warped bases in my case. I was disappointed the Alien Swarm only contained 1 alien, 1 marine and 2 eggs, especially as the ones I received were all in the base box set. Apparently these are all random packs with 4-8 figures and could include the other marine characters, facehuggers, aliens or the Queen, but I doubt I'll be buying another of these 'massive' swarm packs to test the theory out.
Pendragon make 10mm Aliens, but as this is not my scale, nor my interest, I cannot comment on these.
