First of all it is important for you to understand that Krishna is the cause of all the Vishnu avtars. He has confirmed in the Bhagavad-Gita that there is no truth superior to him. This means that He does not have a origin and is the Absolute Truth. All the three Vishnu incarnations; Maha-Vishnu, Garbhodaksayi Vishnu and Ksirodaksayi Vishnu come from Krishna. Even the Narayana expansion etc all of them also come from Krishna. In other words they are the same as Krishna but yet their origin is from Krishna. When you are a devotee of Krishna and offer obeisance to Krishna then you have already worshiped all the incarnations as Krishna is the origin of all the avtars. The Srimad Bhagavatam states all the pastimes of the incarnations of Krishna. The book also talks about Lord Buddha whom the Buddhists worship. It also talks about the Jain God Lord Rishabhdeva which is mentioned in Canto 5 Chapter 6. All these are mentioned as the incarnations of Krishna. Devotees of Krishna derive transcendental pleasure, not only by reading His pastimes but also by reading the pastimes of all his incarnations such as Lord Buddha and Lord Rishabhdeva in the Srimad Bhagavatam. It is also important to note that Krishna has said, whatever we think at the time of death we will achieve. If we are fortunate enough to think of Krishna at the time of death then we are transferred to Krishna’s planet Goloka Vrindavan to live eternally with Krishna. If we think of Krishna’s other incarnations such as Narayan, then we are transferred to the planet where Narayan lives. Similarly all different incarnations live in different spiritual planets of which the chief is Goloka Vrindavan, where exchange of love is the highest. Due to this reason devotees of Krishna think of Krishna 24 hours so that they can also think of Him at the time of death and go back home to be in the eternal association of their father, Krishna.