Use Gargraves Track (FLEX!) and Ross Switches--YES Ross Switches are more expensive, but are ABSOLUTELY the best!!! Second to none! If you buy something else you will be replacing those with Ross later. Save yourself the money. Ask me how I know. The vast majority of serious experienced modelers use Gargraves Flex Track and Ross Custom Switches! Yes...there is a reason why.
GARGRAVES AND ROSS BOTH HAVE TUBULAR RAIL--forget rail nippers!!! You will require a dremel with at least a 1.5 inch cutoff wheel--that is sharp. You must NOT crush the ends of tubular track--pins won't fit.
Yes, you can mix and match---Atlas has solid rail. MY OPINION--I do NOT like the look of mixing tubular and solid rail. And, Ross Switches were made specifically to mate perfectly with Gargraves track. And, a piece of Atlas Flex track (made in China) costs $27! Yes $27!!! A Gargraves (made in the USA!) flex strip costs about $10 bucks. I will let you decide if want to pay 2.5 times more per piece of flex track. :-)
If you get Gargraves, consider the wooden ties and "phantom" (blackened) center rail--looks like 2 rail track. Ross Switches have wood ties through each and every switch. Gargraves has switches on a solid plastic base which can warp and cause the throw mechanism to get stuck. Ask me how I know.
SPACING: 3.5 inches on STRAIGHT track. If it is a place where you will physically couple / uncouple cars: 4 inches so your fingers will fit.
Impossible to give you a precise answer for curves. Depends on the length of the rolling stock and sharpness of curve.
I've given you to much to digest already.
