1) With buying booted, is it best to buy 1 shoe size larger? Normally yes, as it will allow you to wear thicker, warmer (Merino wool) socks and even a pair of disposable foot warmers in the colder months.
2)What's your fave gear to wear under the waders??I like sweatpants. However, when it's cold out I like a thin pair of base layer pants with a thicker pair of fleece pants over them. Take into account I prefer to wear regular,baggy type waders. Neoprenes are generally warmer to begin with.
Best advice is to go to a store and try them on before buying.