I had originally planned on getting only a single GA figure, but I'm a bit disappointed in that one because of a lack of ability to hold the bow or arrows
It's a really nice sculpt, but you can't do much with it really. Kind of pales beside Hawkeye really.
Or does it? Oops. I've been so busy packing things up that I never really looked at Hawkeye or Green Arrow side by side. First, GA will hold his bow, but not an arrow, so grant a point to Hawkeye. But pulling it out reminds me that the loose joints on ML can be annoying and the figure is remarkably small. I'll get more into it later, but ML isn't without it's scale issues as well. I'm also noticing a dramatic difference in accessories. DC Direct stuff is pretty sparse, but when it is present, it's a very hard, stiff plastic whereas ML is quite pliable.

Incidentally, I threw in the Ross version of Green Lantern, in case Jeff is being forced to follow along at home. It's a great figure, but I think a bit larger than the GL series of GL figures.

Brent, if you get a chance, can you take a pic with Supes and one of the characters that would be around the same size (i.e. Colossus, Capt. America, etc.)?
LOL, you have no idea what you were asking, I really had to dig these boxes out of the bottom of the pile. The figures, en masse, are so heavy I had to put them at the bottom of the pile of other figures. Either way, I dug them out. I apologize for the picture quality btw, I know it's poorly lit, but the computer room doesn't lend itself well with the lighting in here. I find the flash washes out the image worse than the crapppy yellow texture of this lighting, so it's a compromise.
Alright, to start off, I stumbled across the Batman that came with the Superman from the twopack and the scale of those, with Supergirl, is really nice. I think the Mattel stuff (DC Superheroes) is pretty nice, but I'm also pretty sure they have only the license for Batman and Superman so they can't do the other JL figures at all. Ever.

So those look just fine together to me. But you asked for Colossus or Cap. Well, how about both? First Colossus and I left the Mattel Batman in there for sake of comparison:

I guess it depends on what you think the size of Superman relative to Colossus should be, but the Ross version (btw, there is a non-variant version that's fairly inexpensive from series 1 if you're leaning that way; the variant tends to sell for about $30 USD). I don't think the DC versions match up too well personally.
As for Cap, well, there's at least five versions of Cap that I know of from ML: Series 1; Series 8; Series 8 variant; two pack version; two pack version unmasked. I only own two of those and if Scott's reading, maybe he can tell us if the Series 8 version is larger than the others. Below is the Series 1 version (on the right) and the unmasked version that came with Strucker (on the left):

I think of Superman being taller than Cap, but that much? I'm not sure. If indeed series 8 Cap is larger, it might be a better pairing. And speaking of that difference, since I've been plowing through these figures lately, I'm noticing that ML isn't quite as perfectly in scale as it maybe should be, considering it's only a single line as opposed to multiple DC line. The discrepancy isn't as bad as between the two Catwomen in one of the initial pictures I posted, but it's still notable:

The Thor on the left is from series 1 (I think, it's the first release anyway) and it's great. It's also larger and IMO a better sculpt than the one from the Giant Man series. It's just a meaner, more godly looking figure. If I didn't have the first, I'd be ok with the second though.
Anyhoo, again my apologies for the yellowish, blurry picture quality. I'd like to spend the time to get better pictures for y'all, but for now those will do just fine for the comparisons that are necessary.