Go for as high a polycount as you can handle with regards to file size and CPU power for slicing. If the poly count is low, in extreme cases you could see those polys in the printed model.
I always design in the scale I intend to print. This is particularly important for smaller scales and small detailed parts. In many cases you will need to take artistic license with the dimensions of things you are printing because of the limitations of the print medium. It will vary depending on the printer, resin, orientation, etc. but you will soon find the limitations of your printer set up. For example, I tend to make any free-standing details, like individual bolt heads, panel gaps, etc. no smaller than 0.05mm. For sheet like surfaces (e.g. rocket or bomb fins, stabilizers, doors) I try to make them no thinner than 0.2mm. If you simply scale down a full scale model in your slicer, you might have problems with the smaller features.
Use whatever works for you. I use Fusion360 because I primarily work on hard surface models, and I find it easy to visualize and build models using the sketch/extrude process. It also lends itself to working off blueprints and scale drawings. Some other people prefer other methods, and especially for things that Fusion360 isn't great at, like softer surfaces such as seat cushions, flat tires, and figures.