It is possible to do so as you would normally with ProxyHA, nothing special is required.
4500c and 4500b have different specs, but the Web Gateway's load balancing algorithm should compensate for any load related issues.
MWGs should always have installed the same version. I think the default value is 3 versions. If they differ to much also the central management is not working any more.
Normally one version should be fine. But i would suggest running both systems with the same MWG version.