Use what's on our wiki, or Memory-Alpha as a guide. If there's no information, yes you're welcome to make up those kinds of details about a species. Just keep in mind that not all species have a "monoculture." Just because we've seen some members of a species onscreen (or heard one or two snippets about them from a computer database) doesn't mean that the information applies to everyone. So an important festival in one country might not necessarily be even known elsewhere, much like Thanksgiving is an American-holiday. (Other countries have their own versions, of course, but the history and food is unique.)
For example, with my character Captain Malcolm Lysander, I chose the Leyron species – whose only claim to fame was having a written language before a spoken language – and created lots of backstory on how their society functioned so that I could tell a more interesting story.
If something is simmed on your ship, and not "disputed" by your CO or anyone else because it doesn't seem too outlandish or out of character for the species, you can probably end up adding it to the wiki if it applies to everyone of that species. We try not to add things to species wiki pages that only reflect one character's information. And we try and make sure everything we add to a species page has a link to a sim if it's new information – helps us keep track of where it came from.
As always, your CO is the best person to talk to if you need guidance on specifics.