You're mixing up sort and search algorithms. Linear search and binary search are algorithms for finding a value in an array, not sorting the array. Insertion sort and mergesort are sorting algorithms.
Insertion sort tends run faster for small arrays. Many high-performance sorting routines, including Python's adaptive mergesort, automatically switch to insertion sort for small input sizes. The best size for the switch to occur is generally determined by testing. Java uses insertion sort for <= 6 elements in the primitive array versions of Arrays.sort
; I'm not sure exactly how Python behaves.