Yes you can however It is not ideal. By Sharing SharePoint you have several concerns, some of which may not be relevant to your environment.
It makes administration harder, SharePoint for example does not need most of hte maintenance processes other systems do, they should be excluded which complicates matters.
By sharing resources you run the risk of SharePoint drowning out your other applications, or vice versa.
You will (probably) not be able to use the SharePoint default Collation or pre-create the databases as easily (fringe effect)
You will (probably) not be able to customise the model DB to better reflect best practices for SharePoint.
You increase risk of users/admins of the other systems gaining inappropriate access to SharePoint content or worse SQL admins who aren't familiar with SharePoint interfering with the DBs directly.
I've seen it done numerous times and it generally works. On the other hand i've seen one badly configured search engine (crawling a Confluence wiki that dynamically generated pages) bring down an entire SQL server. Performance is probably the most obvious issue but in the sort of small shops where shared SQL instances are likely it's also not likely that you're going to be running that much through SharePoint. YMMV of course.