Absolutely! Both classes of products can coexist in one shop. The only disadvantage is that, in case you're using Shopify, you can't use the "override shipping settings" feature. This is because we would not be able to return the correct shipping rates if your customer's cart contained both Teezily Plus items and non-Teezily Plus items.
If you offer both classes of products on your store (regardless of e-commerce platform), Teezily Plus will only fulfill the products that are synced with Teezily Plus. After the product has been shipped, Teezily Plus only updates the shipping status and provides the tracking number (for those platforms which allow it) for the items that were fulfilled by Teezily Plus.