It kinda bugs me when people worship or venerate a Kemetic god(dess) and also worship or venerate another god(dess) from a completely different pantheon. From my experience, the Netjeru don’t really like that. I know that speaking for Them isn’t exactly the best idea, but I was mostly just curious what others had to say about this.

Look into history.

First, egyptians accepted foreign gods and goddesses into their pantheon as well, and syncretized them with Netjeru when it was applicable.
So, they worshipped some forms of Baal, Astarte, Qadesh, Reshep, who came from nearby middle-eastern countries. On southern borders, Netjeru successfully shared the attention of worshippers with local deities of Nubia.

Then, the hellenistic times. When greeks came to Egypt, generally the religion took several equally important directions:
1) Recognize foreign (greek) deities as some forms (=“avatars”) of egyptian deities. Syncretism at it’s finest. Thoth-Hermes as most famous example.
2) Accepting foreign deities for worship.
3) Making good PR for Netjeru, to spread the cult overseas (remember the cult of Isis successfully spreading across Roman empire; miracle-making Khonsu statue being brought to Syria to heal the princess)
4) Advance theological thought to some new divine forms for worship (State-supported cults of newly-introduced deities, look as Serapis as best example)

Some pantheons, indeed, don’t get along pretty well. Especially if they are focused on very, very different highest agendas. As Netjeru are mostly about maintaining Ma’at, they will not get along well with the pantheon that breaks Ma’at for their agendas.

But in the case of Hellenic/Egyptian, and also Roman/Egyptian, the pantheons are compatible, and this is historically-proven, legit fact.

For modern worshippers, it’s also very individual. There are no restrictions.
For some people, the Netjeru may not approve working with certain deities and certain pantheons, by whatever reason. And by the same time, another devotee may work very well with deities from different pantheons.
So if the Netjeru you work with, do not welcome your working with other deities, – this doesn’t mean that for another person it would be the same.

In my own experience, then, the Netjeru are perfectly okay with me paying respect to greek and roman Gods, so I can call myself “hellenistic”, coming from “egyptian side”. I don’t make rituals and offerings in traditional Hellenic way, but it’s okay to honor Gods who are greek/roman by origin.
With Norse, – I “respect from the distance”, not giving active worship.
Hindu deities receive my signs of admiration; but again, I’m a guest on this territory.
I may take part in hindu puja, if it’s open; but probably I won’t go to religious ritual thrown for norse or slavic deities.
I stay away from Slavic pantheon and some other: they don’t resonate with me, and I don’t feel drawn to them.
It’s individual, anyway. I love polytheism for FREEDOM. You choose your path, you venerate and worship Gods you like, you build your own relationship with the divine, and there’s no “polytheistic Pope” to tell you with authority that your devotion is wrong.