I believe that this is completely incorrect. AIMster had nothing to do with AIM messaging. AIMster was trying to do the same thing as Napster did, and they met similar fate. The owner of the company is local to where I live. The "AIM" was because his daughter was named Aimee, and she was prominantly exploited on the site with her pictures everywhere. AOL may have joined in on the legal problems that led to its demise, but it was the music industry that were leading the charge.
As far as risk goes, you need to do the checking with your own legal sources. But, I think you can look at the history. There are a TON of sites with "AIM" in their name that are AIM community sites. I don't know of any of them that have been attacked by AOL. AOL is in hot competition for share with Microsoft and Yahoo, so it would be silly for them to target and attack the community to which they are trying to grow. I see very little risk with this. Further, how much is at risk... for $650 you could just shut it down if they really attacked you.
To the contrary, Ebay has gone after ANY site with "ebay" in the url. It would be suicide to try and run a site with such a url.