The legs are very small on Vulcan. If its intentional, its not working. It looks like you just threw his torso onto a much smaller man's legs.
I second Zach's edit. It makes the colors actually interesting; no offense. The color scheme is just almost the same as your last piece(s). It doesn't look that much darker, and you don't have to be so specific. You change his face, yet you keep his skin lighter? I don't get it; you might as well go ahead and consider the edit, especially since you said you are trying new things, right?
The gun is likely realisticly that size, but people aren't realisticly that size either. So, I think the gun could have more bulk; showing the strength of Vulcan. I would make the barrels (?) thicker, and the guts ( I really don't know gun terms ) smaller.
I would also pull his left leg in a bit; otherwise he would fall. His chest could be pushed forward a bit, too.
It is good, but all over the place.
The only problem I have with Mantis is his other foot not showing; it looks like you hid it just because you didn't feel like drawing it. Since you say you are trying new things, fuck with the pallete a bit, and change some hues after you finish, please.