Yes, but only if they're packaged separately. PKGBUILD is easy, so it takes very little effort to repackage older library versions under a new name (and patch dependents to use the new name) if you need them.
Not really. There are a few applications that can be installed because the install path different for each version, things like Java support this. But libraries like glibc, etc. are tied to one version so anytime those libraries change it triggers a rebuild of many packages.