I'd keep it to a single format though there is no reason I can think of why you'd *have* to. But it'd be better for consistency.
Yeah, that's what I was thinking of consistency.
I would like to present you some os these:
(if you want them u need only to give credits to the autor, that all what he asks. You can find more on kolobok.us and maybe he can make some custom for you!) i think they are very cool and always using them on my smf forum, and they work pretty good together... just try. i know that they are not made by me, just givving you to know, and maybe you dont need any contest at all?
not sure...
Not to put down this set at all, but while it looks very good against a light colored background, try throwing it up against a dark background and you will see all of the many imperfections in the borders of these smilies that will show up like a big sore thumb. The problem with these smilies is that they use a white matte instead of none. While the white matte preserves the original border around the smilies a bit nicer, you sacrifice having it only look good against certain backgrounds, in this case white and very light colors will do okay (which IMO is a huge sacrifice that isn't worth it). If you change the matte to none, you will get a fully transparent background for the smiley, which will most likely make the edges a bit rough, but will look good against ANY background color, image, etc.. So, just my 2 cents on this, is better to make the matte = none.
Cheers

EDIT:
For example, here is the grin smiley from the set, that you found elsewhere online, with a dark background (and trust me, is the same for all smiley's that you posted above)...

And here is my laugh with the same color background (took a still snapshot of it, mind you)

I'd rather have my edges look a little rough than to look like that!
RE-EDIT:
There is also the other possibility that the creator of these smilies purposely placed in the white around the smiley's in order to maintain the smooth border appearance in light backgrounds, with a matte = none, which would give the same effect. However, in any case, you see the problem with dark backgrounds, not worth it IMO.