Second monitor for my Dell S2417DG

[SimoneP]

I have a Dell S2417DG for a few years now and I'm really happy with it.  
 
Now I'm looking to get a second monitor to use for browsing/discord while I'm playing games.
 
I've mostly been looking at Dell monitors that have the same bezels/stand and I'm deciding between the following ones:

* Dell P2418D (€224)  
* Dell P2419H (€156)  
* Dell U2419H (€195)  
* Acer Nitro VG240Y (€150)
* Acer Nitro VG240YU (€204)

I've read that having a 60 Hz and 144 Hz screen together can cause issues, especially while playing games. Can I plug in the second monitor to my onboard graphics (i7-8700K) and the other one to my GPU (GTX 1080Ti) and will that fix those issues?
   
I also read about G-Sync not working correctly if you have two monitors and play in borderless windowed. Is there anything I can do about that?
   
Also do I need to go for a Quad HD monitor because my main monitor is also Quad HD or does that not matter? I'm worried that everything won't scale well if I go for full HD.