Is it possible to use Thunderbolt networking and regular ethernet networking simultaneously?
Yes, that is definitely possible - indeed this is what happens if you hook them up like this.

Thunderbolt 3 enables transfer rates up to a theoretically high of 40 Gbps.

The relative speed depends on the type of Ethernet connection you're comparing it with. If you're using 10-gigabit ethernet - then you're probably not going to see any practical benefit from the Thunderbolt 3 connection (if you're not transferring stuff from disk, you might get huge benefits though). If you're using gigabit or slow ethernet, you're going to see a massive improvement in transfer speeds.