Yes, I think Peter Dinklage would be pretty awesome in an Expendables movie, but put him alongside Stallone, Statham, Lundgren, etc. and you'd struggle to have him seen as anything but comic relief if it were the first movie and you're still selling the concept. Put him in the third, or fourth, movie, and you can use him to mix up expectations. Putting him in the first movie's trailers, however, would be a much more difficult sell.
I, personally, don't think that the movie should have a 100% serious tone, but movie audiences have a really strange relationship with SciFi, and the public doesn't react well to SciFi that mixes comedic elements into it's seriousness. Maybe it's too much association with B-movies, but it's hard to get audiences to see that comedic elements can work as well in SF as they do in buddy cop movies.