There are apparently some folks out there who would insist that CR can only be practiced by itself and that a CR person can never honor any deities from any other culture. I think this is bullshit, pure and simple. I believe that, as polytheists, it's our responsibility to properly honor any deities or spirits who come knocking, in their proper context. In some cases, if your home is small, you may only have space enough for one altar, and in those cases, it's okay for deities to share space even though you're honoring them separately. If all you have is a windowsill for your altar, that's all you have, and it doesn't make you not-CR if you have Thoth on that altar beside Lugh and Epona and Dian Cécht. We all know they're not the same deity and they aren't approached in the same ways or with the same ceremonies and offerings.
Eclecticism and syncretism are kneejerk words for a lot of people, myself included. This is because a lot of us have seen them used as an excuse for the worst possible mashing together that ends up with things like thinking Kali and the Cailleach are the same Goddess because they're "crone Goddesses" (which is more bullshit) and their names sound similar. This is right out of the Barbara Walker school of confused monism. This is not eclecticism or syncretism being done properly. It's laziness and self-delusion.
If we look at the definitions of eclectic and syncretic, we can see that both are necessary if we're going to reconstruct, recover, or otherwise (re)create a group of modern Celtic paths that are full and workable in our time and for our lives today. The little hints we have about the internal cauldrons that are a part of Gaelic filidecht are going to be nearly impossible to reconstruct without some reference to Hindu understandings of chakras, despite the fact that the cauldrons and the chakras are obviously not the same thing. We don't have enough in the Gaelic material to go on without those outside references. People on a CR warrior path inevitably seem to end up studying eastern martial arts at some point because that's where most of the information is. This is slowly changing as more information becomes available about western martial arts, but it's still a basis for a lot of the modern CR warrior path and serves as an important point of comparison.
CRs can legitimately and happily be a part of religions other than CR. Polytheism recognizes all the deities and spirits, even if any given polytheist doesn't honor all of them. Honoring all of them would be impossible because there are so many, even within just the Celtic cultures alone. Remember that historical syncretisms occurred -- we have Romano-British deities, we have Gaulo-Roman deities. We have Epona who was honored by Romans. We have the mixing of cultures in Ireland and Scotland where the Norse/Germanic peoples came in and shared space and families and cities with the Gaels. Celtic cultures extended from Iceland to Spain to Anatolia, and ignoring all of that in favor of some sort of Gaelic purity movement simply does not make sense.
Pay attention to context. Pay attention to the deities who come to call on you. Respect each of them for who and what they are. Don't turn your back on one just because it's not "Celtic enough." Live your life with honor, and have fun.