Yes, You can add custom fields to your product if you are familiar with editing theme or variations is another option to do same thing.
You can share url or explain in detail what you want to achieve I can help you with it.
For this case where you want to keep track of inventory as well, I will recommend you to use product bundle app,
That will help you in keeping inventory contorl as well, and you can allow customer to select as many addon they want to get with your product,
here I used product bundle, and each product is separate product in shopify to track quantity.
Order form customization is the simplest straightest method , it just needs to be adapted to have a limit and show the items together in the cart
There's also the alternative if your scents are single smells, thus single variant options, then just make those combinations as a prebundled product then use a sync app to process inventory levels on skus.
There's also the possibly utilize the buy X get Y discount in specific bundling scenarios