extra (medium sized) thumbnail creation

I'm not following you ?
In any HikaShop module and menu, you can set images width and height for that menu/module listing in the HikaShop options of the menu/module. Is that what you want ?

You could just create a menu item of the type hikashop products listing accessible only to "special" user groups and set the size you want in the hikashop options of your menu. Then, access the menu on your front end and the thumbnails will be generated automatically.