The description can be viewed in a slightly different manner.
Kashyapa is a person whose job is to regenerate Earth and then populate it. So we will find different wives and different progenies including dogs (Sarameya from Sarama), River (Iravati) and so on mentioned at different times.
Consider the "Snake" described here as Naga people. The thousand "snakes" are just small but powerful groups of Naga, or may be their kingdom.
Same way, the Garuda and Arun are not birds but kingdoms.
Same way, Kadru and Vinata etc. are metaphors…not necessarily a human being.
There is another story about Indra and Vayu. Vayu's mother was asked to keep vigil while Vayu was in womb. Somehow she fell asleep. Indra went inside the womb and cut the embryo in 49 pieces. It can be realized as a Kingdom in making….jealous Indra divided the people in 49 pieces !
The basic thing is, in most places metaphors are being used to make it easy for people to remember. These Itihasa and Purana were for common people. Veda and Upanishads were for learned people. There metaphors are less, but sometimes more cryptic.
Without understanding metaphors (which, frankly speaking, most of us do not understand), literal study of any epic will create confusion. It is like a English Professor reading a physics book on Quantum mechanics !
regards