You can have a mix of Norse, Greek, & Celtic if you are called to work with them.
I have never been drawn to or anything of that nature to anyone outside of Irish Celtic deities. Both Brighid & The Morrigan. My practice is heavily Irish Celtic based.
We have members that work with multiple pantheons. I believe @Mistress_Of_Herbs works with Hecate History, Theology, Mythology, the Morrigan, & Thor.
I believe @Amaris_Bane works with different pantheons also, but I could be wrong…
Some members work with deities based on their needs or correspondences for what they are doing that day or the spell or ritual.
Whatever or whoever feels right for you to work is perfectly fine. Remember to be respectful & if you choose to work with them, learn their history & stories… what they may like for offerings & meditations 
to meet with them & see how the relationship will be or what’s the best way to connect with them. 