Hi,
The cart system does allow you to have different sizes (variants) of one product to reach that 10 minimum quantity.
However, I think the issue you have is that the quantity input field of the product defaults to that minimum quantity and doesn't allow you to go below.
But if you add different variants to the cart (each time with a quantity of 10) and then reduce the quantity of each variant to get a total of 10, it should work fine.
So in that case, what you could do is to configure the "Add to cart on listings" setting to "global" and the "Characteristics selection method" setting to "list" in the HikaShop configuration.
That way, on the product page, you'll have a listing of the variants of the product in a table, with one row per variant, and each variant will have its own quantity input field with a global add to cart button at the bottom of the table.
So if you add at the same time different variants with a total quantity of minimum 10, it should allow you to do it, but if the total is less, it should prevent you from doing it.
That's the only way I can see to achieve this without any development.
Now with the development of a plugin, what you could do is leave the minimum quantity to 0 so that you could add any variant of the product with any quantity to the cart at different points, and in the plugin, add a check before the order is created to cancel the order creation if the total quantity of variants of a product in the cart doesn't reach that quantity of 10. And you could do a similar check when the cart is being loaded on the checkout to display a warning (so that the customer see the issue at the beginning of the checkout, not only at the end).
