In a situation like this, I find it helpful to ask two questions to help make a design decision:
How problematic is it for a user who expects a certain behavior, if that behavior is not there?
How problematic is it for a user who does not expect a certain behavior, if that behavior is there?
I strongly feel that collapsing (or "cropping") comments is bad design in most situations. And it's exactly because of those two questions above:
If I do want to follow along in a comment stream and actually read most of it, I don't care much whether the page looks "neat" (in the purely aesthetic sense, mind you). Having to click
Read More links to expand a lot of collapsed comments, though, quickly becomes tedious and distracting.
If I do want to skip a comment, well, I can just scroll ahead to the next one.
If I do not want to follow along in detail and just skim the conversation, scrolling ahead will work just as well.
In other words, the extra burden of having to expand collapsed comments for someone who does want to read them is far worse than the burden of having to scroll and manually "hit" another comment.
So, instead of collapsing the comments, consider making sure that the stream as such is easy to navigate: making the comment headers (with their author's avatar, etc.) stand out visually will allow visitors to easily make out the beginnings of a new comment even when just resorting to using default scrolling.
If you do insist on collapsing the comments, though, try providing a
Expand All affordance, to reduce the interaction overhead to a minimum for those visitors who do read 'em all.