Does the listing show compatible plugins with the forums version you have installed? I saw sso-github twice in that list so I'm wondering.
Right now it doesn't. @psychobunny is doing some work on the package manager. You can check out the discussion here. Eventually that list will display only compatible plugins with other useful info.
There's no baked in way to get that kind of permissions control. It's either all-or-nothing when it comes to the ACP.
Aw, David Tennant! I just finished watching The Fall of Pompeii
is there a setting to set default size for embeds?