Visual designs for multiple screen sizes

We are designing a website for desktop and tablets, which needs to work for 1366x768, 1024x768, 1920x1080, 600x1024, 1600x900 - the most common resolutions as per Stat Counter Global stats in India. I understand responsive design with break-points is required.

However, in the visual design stage (Sketch/Photoshop), we are only creating visual designs for 1024x768 resolution, as this is perhaps the most common approach at the design stage.

However, I am concerned about two things:

  1. Screen resolutions are already increasing. How do I ensure my website design is scalable to work for newer high pixel density technologies (example: retina display) or for large screens. I read up about fluid designs which will address higher resolutions, but is there any specific point which needs to be taken care of? Should our design be responsive to viewport changes or screen resolution changes?

  2. There are multiple aspect ratios we are targeting with a single design. Is it a better practice to design differently for say 1366x768 and 1600x900?