Any of the attinson's products have small pellets. Also NLS betta pellets are small.
A fish is a marble if it changes color at all. There is red loss which is something different but on any color fish a marble is one that gains or loses color. Butterflies are also a form of marble that don't change color.
Two weeks is the farthest I'd recommed going with out food. Even then the betta's immune system starts to be lowered. Technically a healthy betta could go a month without food. (NOT RECOMMENDED)