Starting with the quick answer first: Axel and Dom share a polyamorous route. Some details will vary depending on who specifically you chose in the beginning, but the broader strokes will remain the same.
As for tales, a general explanation for this is in our FAQ. We've got a series of stories planned and how much we end up doing will ultimately depend on interest in Sileo following our 1.0 release, but a simple explanation is that it'll contain stories that don't fit within the main narrative, either due to occurring before or after the story, or during from a different perspective. An example of each: One tale will detail some of the shenanigans Adrian, Diego and Tobias got up to at university. One tale follows on directly from Diego's route and has you meeting his family and exploring his hometown alongside Diego. And one tale explores the events that lead to Jay and Eric becoming a couple in Tai's route from both of their perspectives. Currently we have more than a dozen loosely planned, though the contents of that list will change before 1.0. We plan to release a few of these before 1.0 as well, the continuation of Diego's route mentioned above likely being the first we tackle.