Hindu Gurus of all Sampradayas are allowed. As said in this answer, these Acharyas might interpret some social norms differently but they all adhere to core beliefs, like Apaurusheya of Vedas. Some of core beliefs can be seen from this answer. Although list of core beliefs changes with different acharyas, many common beliefs like believing in Vedas, reincarnation can be observed. For instance, Sri Sri says,
Every sound has a certain effect on the consciousness. For example,
when we hear some pleasant music, it calms us. Other types of music
make us get up and dance - even without knowing the meaning of the
lyrics! In the same way, the ancient sages, called Rishis, going deep
in meditation, researched the effect of different vibrations on
consciousness. Assimilating these different sounds, perceived in deep
meditation, the Rishis came up with mantras, which have a specific
impact on consciousness. During Navaratri, various homas are performed
and mantras are chanted which have a harmonizing impact on the
consciousness, as the Rishis described in the Vedas.
These seer-scientists were called 'seers,' because they 'saw' the
knowledge. They did this by going into deep meditation and quieting
the mind. In such a state, the Rishis could understand the
characteristics of the forces of nature. Their calculations and
mathematical methods are accurate even today. Thousands of years ago
these seer-scientists had already described many things, which Western
science has discovered only in the last few centuries – an example
being the description of molecules and atoms. Observing nature, the
Rishis could describe the energy or consciousness that is pervading
this whole creation. After passing on their knowledge orally for
generations, it was finally written down in four main scriptures of
His views on core scriptures Vedas are in accordance with tradition. Swami Vivekanada here gives different list but we can see many common things like belief in Vedas as truths, existence of an entity which is eternal and all pervading.
Obviously different acharyas or Gurus have different opinions on topics like Varna Dharma, homsexuality and they do propose their views only after interpreting scriptures.
So, in short we can allow Hindu acharyas and even Scholars or Indologists if they don't (list is not exhaustive),
- ridicule Vedas as mere imaginations of some "poets"
- dismiss law of karma
- reject existence of one all pervading entity (can be called by different names like Purusha, Soul, Atman, Brahman, Shiva, Vishnu, Shakti based on sampradaya).