Is it possible to change the way Kubernetes assigns IPs to pods
Technically: yes you can either configure or edit-the-code of your CNI plugin (or write one from scratch).
In practice: I know of none that work quite that way. I know Calico does allow having multiple IP pools so you could have a small one just for Selenium pods but I think it still attempts to minimize reuse. But check the docs for your CNI plugin and see what it offers.