1. Every parent object in Saleforce is Chatter Feed enabled. By default, the standard objects have been configured to create Chatter events (or feeds) as result of creates and updates (of certain key fields). To make changes, go to Setup>Customize>Chatter>Feed Tracking. You can configure feed tracking for each object (custom included) to track changes to standard and custom fields. For more information, check out this link.
2. Any existing external data integration into Salesforce can instantly be Chatter enabled. Simply go to the feed tracking page for the object of interest and enable tracking for the fields you're updating.
3. For child objects like the Opportunity Line Items, you'll notice that there's no feed tracking capability. This makes sense as you would typically want to track an Opportunity as a whole and not just individual line items. If you're updating the line items with external data such as a "Ship Date" from your ERP, add an extra step to your process to create a Chatter Feed post for the line item's parent opportunity. It's a simple Create call to add a single record to the FeedPost object.
4. Avoid creating additional custom objects and synchronizing external data if all you're interested is being notified of updates from your external applications. It not only creates more work to create and maintain the integrations, it will also require users to subscribe to additional data objects. If you only need to present data from the external systems to the users, consider building a quick mashup instead.
5. Avoid creating a User Update or Post on behalf of an external system as it will potentially bypass the security rules you have in place and may expose sensitive data to your entire org. Since users can only subscribe to Chatter events for data they have access to, it would be a best practice to create feed posts associated with specific data objects.