I am aware of problems with the smelting configuration and they will be fixed with the next update. If you need advanced control over the smelting process (tick speed, etc.) I highly suggest you to use a plugin that is specifically designed for that.
I think you misunderstood the use of categories. Categories are used to create a pool of items where a configurable amount is selected. 6 Categories should be more than enough for that. If you want 1 or more items to always spawn along each other, configure them as extra items. Extra items are not counted in the total item amount setting.