Sunrise,
Personally I prefer 18” passenger cars. Currently all Lionel’s streamlined passenger cars are 21” but they did produce 18” cars in the past mostly in aluminum. They still produce 18” heavyweight cars too. You may also want to look at K-Line 18” cars some are currently in the buy/sell forum in streamlined, heavy weight etc. They made all lengths of passenger cars make sure you look at their last produced cars so you get detailed interiors and passengers. I would not go smaller than 15” which also would work with you curves and still be scale. Lastly MTM produces 18” passenger cars in their premium line. Their cars are generic but their painting is terrific and in many road names. If cost is an issue you can look at Railking’ s 15” cars too. Good luck
JohnB