My understanding is that you don't actually post "as" the community, because it's... well, a community. It's a common place for the individual members of the community to post. What you might be thinking of is an LJ syndicated account to which people can subscribe -- it's like an RSS feed for your LJ. We set one up for the other site I work on, it looks like
this. As for using the extra icons... I'm not entirely sure, unless it means the icons that are displayed at the top of the community's page. For 5MV, the icon in the top right -- that's probably what you have room for 15 of. Which is dumb and pointless.