Your rig should work with the Acer Predator XB270Hu, as it supports that model.
Your rig should work with the Acer Predator XB270Hu, as it supports that model.
Well, if the configurations are low enough, yes. The key is: 1440p is roughly twice the pixels of a 1080p screen and runs at 144hz, which is essentially more than double the usual 60 fps. You'll have to increase the frame rate by about two while also increasing the pixel count significantly. That's quite a challenge for just one 780 monitor, so you should reduce your settings (especially for Unity, I'm not sure about LoL or CS).
Your choice to use your old screen as a second monitor won’t affect your gaming performance. Watching streams or videos shouldn’t cause any issues with gameplay.
Streams or videos could cause a slight slowdown or frame reduction, but it won't be noticeable during gameplay.