Three different types of soldiers come in legion should I customize it with one type of soldier?

You can try different arrangements of troops to see what works best for you. A mix of all three is a good start but as you level up you will be able to add buildings called war rooms that will give you buffs when all of the troops in a legion are the same. In this case it is a good idea to do this.

It is also a common tip to use all cavalry in a legion to increase the march speed as they have the fastest march of all troops.