Yes you can. As long as there is enough space.
CrossfireX is only required if several AMD graphics cards are to work together.
What you should look for is if you have a free PCI-E 3.0 x16 port, and if your power supply provides enough power and connections.
Greetings

Forget it, the 2070 series is not SLI capable, so be careful!

Yes, you can. You simply cannot use two nvidia graphics cards in SLI, which is not possible with this graphics card anyway.