After installing version 0.15 (indifferently on amd64/x86) plugins 0.14 continue to works, but some of them have no icon. Upgrading the gmpc plugins to version 0.15 (not yet in portage) solve these problems, anyway imho a post-install warning can help.
You're right - I need to update plugins in the tree. Will do that in a day or two, depending on my work load.
i tested the new plugins (on amd64)... a simple rename of the ebuilds seem works without problems. good work.
Looks like this is fixed, closing.