Srila Prabhupadai's writings are not considered to be authoritive Shastra, like the vedas and puranas. His writings are only interpretations. But even Srila Prabhupada accepts the Puranas as authoritive works, often quoting from the padma purana and the brahmavaivarta purana to support his arguments. If you would study these two puranas, you will find that there are two classes of people mentioned who are not considered vaishnavas. Not even by Vishnu himself. These are first, the person who regards Shiva and Krishna as different and second, the person who regards Radha and Krishna as different.
Not only are they disregarded as vaishnavas, it is also mentioned that their right are the hellish planets after death. Shiva is Vishnnu, and Vishnu is Shiva. Radha is Krishna, Krishna is Radha. If you cannot accept this, you are only pretending to be a vaishnava. Study the Puranas thoroughly and you will see that the ISCKON philosophy has no foundation in the Hindu Shastras. Bhaktajan, Bhakti is like a ripe fruit. You have heard about it, you have talked about it, you have seen it, you have smelled it, but I promise you, Bhaktajan, you have not tasted it, not yet.
Bhaktajan, I hope you are capable to think about this and maybe even curious to do some research and study the puranas independant of Srila Prabhupada Ji's interruptions. Followers of an Acharya might be stubborn and never change in opinion and belief. But their children and grandchildren will not understand these stupid quarrels between devotees and therfore convert from Sanatana Dharma to other religions. "Allah is the supreme and only God, no debate." and "Jesus is our saviour, no fighting about that." They will regard Hinduism as obscure, only because they were deprived from the essence of it.
The Sanatan Dharm has such a beatifull and rich philosophy. But these acharyas in Kali Yuga mislead their disciples and tell them not to read the puranas. They'll tell you it will only confuse you. They want to keep you in the dark. I will only say this. If your intellect is small, you will get confused. Yasya Svayam Nasti Pragya, Tasya Shastra karoti kim? But if you are intelligent enough, you should atleast read the Markandeya Purana, the Bhagavat Purana, the Vishnu Purana, the Shiva Purana and any other Maha Purana by your choice. Read them in their original language if possible, or read an uncoloured translation. Then you can make up your mind about Shiva and Vishnu, not before. Not without knowledge.
AUM namo bhagavate rudraya vishnave mrityor me pahi.