Welcome to Bluelight!
Hope I can answer your Qs:
--The hole on the side is a "carb"--you hold it when you're hitting the bowl then release and pull through to stop the bowl burning
--You can add a filter if you're worried about pulling through--either a "glass filter" which is like a little glass jack that fits in the bottom---or a metal filter which is wire mesh and would go at the bottom
--You can smoke hash with this pipe, but I'd recommend a metal filter so it doesn't get pulled through. Just use indirect heat on the hash
--You don't have to mix tobacco into this bowl, but if you wanted to smoke your hash in a joint you'd prob mix it with tobacco (or flower cannabis)
--A bigger bowl will probably waste more smoke. With a small pipe you lose very little. Though if you're smoking hash a lot you might consider a small metal hash pipe (has a cover for the bowl)