This shouldn't be an issue, because the excerpt is "calling" the content from the correct page, and it's on your own site. If you were quoting large amounts of text on the main page rather than just calling a section of text then it might be seen differently. I would make sure that I'm simply showing a section of text from the full article, and not copy/pasting it in.
I'm not sure what you mean by IM. But in WordPress and others the method is to call content from the right page to display as a snippet on a main page. The content is still contained on one page, it's just presented as a link would be with a description. This shouldn't affect your SEO or the way Google views the site. As mentioned, I wouldn't be quoting entire chunks of text on several pages, but calling a small portion of text to be shown on another is not a bad thing.
I primarily use WordPress, and this allows you to have the title, such as "This is a post about blue widgets" while changing the url to be shorter, for example "http://myblog.com/category/blue-widgets". This is something to consider if you can do it, maximising the impact of the most important words in the url and keeping it short. The general rule is the shorter the better.
Don't cram the url with keywords, and don't fret about the title of a post either. Keyword use in a title is less and less relevant these days (others will argue this). I have seen one client of mine who requests all of his post titles be scene titles where a keyword is not present, and as long as I write the post to target something it does well - perhaps even better - than those with the keyword in title.
This is a usability issue rather than a Google/SEO issue. But then I guess if it makes the user experience better then it can be considered an SEO issue.
Think about the common method, with images usually linked to the content. It makes sense that if you have a page of posts and they all have an associated image then users will automatically try to click the image to access the post. I started an adult blog last year with a theme that had post images and a text excerpt on the main page, but the images were not clickable, so I changed the code to make all images click through to the associated post.
While it might not affect Google in any way (there is no real concrete advice about clickable images that I have ever seen) it does increase usability and it will likely increase the depth of clicks and reduce bounce rate slightly with those who just expect all images to be clickable - which is almost everyone on the net these days.
These are just my opinions. I am by no means an expert, but I have been doing this for about six years now and creating content for others is what I do for a living. I hope it helps