Well from personal experience, even on a hard tail there is always some chain slap. A few things though, first, is your chain cut down to the right size? If you don’t know how to do this there are lots of great videos on youtube like bike radar to show you as well as i believe gmbn, if you are not comfortable doing that your self take it to a bike shop to make sure, but i suspect it is the right size unless you put on a new chain yourself. Another thing that helps with chain slap is to have a rear derailleur that has a clutch system in it. That allows the derailleur to not move forward when going over bumps and therefore keeping some of the slack out of the chain caused by the chain pulling on the derailleur. The final thing is use a chain stay protector to help protect for scratches on your bike. To answer your last thing, being that you are running a 3×8 i would pass on the chain guide as it doesnt really gain you much as there will still be some slap. Anyways hope that helps.