Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
There is no objection to mix Zam-Zam water with other water and present the mixed water to guests. But the Baraka (blessing) of pure Zam-Zam might be greater than the Baraka of Zam-Zam mixed with other water.
So, it is better to drink Zam-Zam pure even in very small quantities since what is sought is Baraka.
Allah knows best.