You can include the path to your partial when you render it:
render :partial => ‘user/my_partial1’
render :partial => ‘comment/my_partial2’
As for the way you’ve describe your app, it sounds like you’re triggering a lot of queries from your helpers. Personaly I like to keep that to a minimum, especially for essential entities such as comments. It’s handy to see from just looking at your controller what data you’re providing. Sure, small queries in a helper is fine, i.e count rows, getting user’s real name etc.
Don’t burry things away to quickly, you may know where they are now and what they’re doing, but 6 months after your site has gone live are you still going to remember so readily?