Before I install the new installers recommended do I need to delete the old versions first
The instructions don't say that, so you can assume it's not required. If it was, we would clearly said so or, most likely, have the installer checking for this and prompting to do it.
and do I need to download the most recent version of the stand alone manager also.
The instructions don't say that, they only say to download and install the latest Full installers for KIAH and GSX.
All the Full installers of all our products contains everything, including the Addon Manager so, if you just download them now, there's no need to install the Stand-Alone Addon Manager too. The only time you would want to download the Stand-Alone Addon Manager, is when you want to update that one *without* downloading a Full installer but, since in this case you must do it anyway, the separate Addon Manager is just redundant.