Xyrillians are one of the first aliens that the U.S.S. Enterprise NX-01 meets, in an episode where Commander Tucker is accidentally impregnated by a Xyrillian female. Xyrillians had advanced holographic technology then, as well as cloaking technology. Although they have appeared only once, there were many things we learned about Xyrillians in that episode that they might make an interesting addition to your campaign. Also, they likely have learned from the Enterprise incident and conduct their interactions with other species more carefully. Non-canon sources have suggested that they became members of the Federation by the 24th century. They may appear in any era of play.
As the Xyrillians breathe a very different atmosphere, it is assumed that they have created technology that will allow them to interact with other species more easily. The Xyrillian Technology Talent is recommended, to deal with atmospheric conditions typically encountered. The telepathic granule game allows telepathy both ways between the two people playing the game.
Their proficiency with holographic technology may also allow player crews of any era to experience a Xyrillian holo-chamber. Given that the Xyrillians installed one aboard a Klingon bird-of-prey, it may be a way to introduce a holodeck experience to any era without an actual holodeck.
I introduced a female Xyrillian in my campaign, an Intelligence Officer from Starfleet Intelligence. I include stats for Lieutenant Commander Rili’L (Notable NPC) below. She may be a good addition to a ship, especially during times of war when an Intelligence Officer is needed to analyze incoming intel or report on enemy movements. Proper accommodations (food sources and high pressure environments) will have to be made to host the Xyrillian aboard a Federation starship, but that could be an interesting problem for the players to deal with.
Click on images for PDFs