Hare Krishna!

In my limited Gaudiya understanding, Maha-Vishnu is an expansion of Lord Krishna. Maha-Vishnu is the incarnation of Lord Krishna in the aspect of Creator, then divided into Kshirodakashayi-Vishnu (Dweller in the heart of every living being, Paramaatmaa, etc.) and Garbhodakashayi-Vishnu (Creator and Dweller in the Macrocosmic forms, universes, etc.).

Worship of Krishna is worshipping all His incarnations and demigods. He is fully sufficient.

Jai Sri Krishna!