Can I use these older products as a node with the Linksys velop as a router?
In theory, yes. In actual practice, not really.
Yes, if you want to use the AirPorts to provide a wireless signal for devices near the AirPorts. Think of the AirPorts as if they are providing a separate network that is not really related to the Linksys mesh network.
No, if you are asking if the AirPorts will behave as if they were an actual "node" or integral part of the Linksys mesh system. Linksys provides no means to monitor the performance and behavior of a third party router on their mesh system.
If you are considering the Velop system, keep in mind that while Apple sells the product, but they will not provide support for it, so unless you have a specific reason for buying from Apple, you might want to shop around.
Here's the fine print from Apple on this:
Note: Products sold through this website that do not bear the Apple Brand name are serviced and supported exclusively by their manufacturers in accordance with terms and conditions packaged with the products. Apple’s Limited Warranty does not apply to products that are not Apple-branded, even if packaged or sold with Apple products. Please contact the manufacturer directly for technical support and customer service.