If l is regular, the number of classes is finite. Let each state in the fsm determine a class. If x and y lead to state t, then xz carries our fsm to a final state iff yz does. Thus x and y are in the same class, and all the strings that lead to state t are in the same class.
Suppose states t and u determine the same class. Let any transition that use to go to state u go to state t. This does not change the language at all. Get rid of state u, and any other states that have become inaccessible after deleting u. This reduces the number of states. Continue this process until each state represents a unique equivalence class. The result is a minimal fsm. If an fsm had fewer states, some state would represent two different classes, which is impossible.
The states of a minimal fsm represent the Myhill Nerode classes, and one minimal fsm can be mapped onto another by using these classes as a correspondence map. In other words, the letter A might take the first machine from start to state g, while the second machine winds up in state h, but g and h represent the same equivalence class, so we can equate them as g ⇔ h. Do this for all states, and note that the transitions are preserved, hence the machines are isomorphic. The simplest fsm for a regular language is unique.