I know JoomSEF built a plugin for HikaShop and we have the sef file for 404 directly in HikaShop so you shouldn't have problems using them.
In each product, you can add related products on the back end. Then, you can go to Display->Views and create a new module. Set it to be synchronized and to display related products on the product page. Then, go the configuration, under the tab display and for the option "modules on the product page" you can add that moduel you just created. Then, you should see it under the product page when you have products related to the main product displayed.