I don’t know what’s up on your case, but I would not jump to the conclusion that it’s impossible to use tailscale with any other VPN in any circumstance.

Rather, tailscale and Mullvad will now work easily and out of the box. For other VPNs, you may need to do understand the topology and routing of virtual devices and have the technical ability and system permissions to make deep networking changes.

So I’d expect one can probably find a way for most things to coexist on a Linux server. On a non-rootrr android phone? I’m less confident.