In the reimagined Battlestar Galactica series, the only species people see on-screen are humans and Cylons; it was a deliberate decision from the beginning that the show didn't include aliens. Based on Glen A. Larson's Battlestar Galactica miniseries from the late 1970s, Syfy's reboot sought to build upon what was established in the original series while also taking the franchise in a new, modern direction. This included a number of deviations from the original story, some of which weren't well-received from audiences and critics.
Among those deviations were the two Earths and how the show ended compared to the ending of the original miniseries. While those were perhaps the most prominent alterations, the overall tone the writers and producers took with the reboot was starkly different from what had come before. Syfy's Battlestar Galactica was dark and it tackled adult themes, religious themes, and became a true sci-fi drama. In order to adhere to those tonal changes, the reimagined series didn't include any aliens.
There are two reasons why there aren't any aliens in the Battlestar Galactica reboot; one comes from the series creator, Ronald D. Moore, while another is a rather odd stipulation from one of the main leads, Edward James Olmos, who played Admiral Adama throughout the show. Moore told Wired in 2010 that he had a list of don'ts from the start, which included not encountering "aliens that control our minds. ...I'd had enough bumpy-headed aliens from Star Trek. I thought the show would be less interesting if it was about that." By not including aliens, Moore and his team were able to keep the story focused on humanity's survival.
If Moore changed his mind, however, Olmos had his own way to prevent aliens from showing up on-screen. He once quipped that he put an avoidance line in his contract that stated he would kill off his own character if they included aliens. Olmos said (via Wired), "The first four-eyed monster I see, I'm going to faint on camera, and you're going to write me off the show." Whether or not it would've come down to that is unclear, but for the betterment of the story, Battlestar Galactica never included any aliens. However, that wasn't the case for the original series.
Larson's Battlestar Galactica miniseries did have some alien species, such as the Boray from the planet Sectar and the Ovions, who were once enslaved by the Cylons on the planet Carillon. While these species gave a deeper insight into the Cylons and the greater universe as a whole, it worked better at the time - in the 1960s and 1970s, sci-fi shows and movies were always focused on finding new life - it was a tired trope in the early 2000s. Syfy's Battlestar Galactica told a different story, and including aliens would've infringed on that.