The Skaal hold all life sacred, to the degree that they hunt only out of necessity and abstain from cutting living wood. They aren't pacifists, but they welcome pretty much anyone who visits their village with peaceful intent, including mer and beastfolk. They have allowed outsiders to live with them for extended periods of time, and while they probably don't think too highly of the Daedra worship the Dunmer practice, they nonetheless trade and barter with the residents of Raven Rock.

If an outsider was willing to adopt their beliefs and customs, I doubt they'd be entirely adverse to the idea. Mer might have a harder time, though, even if the establishment of Raven Rock probably made them more receptive to the idea.

As for beastfolk, the only example of them interacting with beastfolk (apart from Khajiit/Argonian LDBs) is Gaddan from ESO, who is a Skaal who resides in Stormhold in Black Marsh. He can be found by the local Hist Tree together with a few Argonians and doesn't really seem to mind.