i think the key question is, do you want to give customers the option of buying cushions ONLY, or must they be purchased with a chair/sofa.
What happens if a customer who already has cushions needs to buy new/ replacement cushions.
I realise I know nothing about your business, (or your datbase structure) so I hope I'm not leading you astray here, but I would be tempted approach this 2 ways
1. Add cushions to the existing products database, and change the menu so that they can be chosen like any other item, (category 8 ??)
This has the benefit that the existing cart structure should still work.
2. Create a 'linked items' table, where you could associate cushions and chairs/sofa's and vice versa. With a little bit of coding you would be able to display related product.
eg on a product page displaying a sofa, you could show cushion/cushions suitable for that sofa
Just a thought :-)