Thread:Project:Support desk/Pictures at with Interwiki and Commons/reply

You probably need to allow uploading but disable Special:Upload on the wikis where you don't want uploading (see Manual:Hooks/SpecialPage_initList).

Also, I think you'll want to use $wgForeignFileRepos instead of $wgUseInstantCommons to point to your own commons instead of commons.wikimedia.org.