Ashtanga yoga. Is it always neccesary to follow a series or can you do random asanas?

There are basic poses, and then there are series. Basic poses are meant to be done for tension release, pain rehabilitation and so on. Like in your clroom or office or after lifting too much perhaps. The series of poses, routines, are meant to flow together to get the best benefit. If you have a routine on a video, follow it. If your looking at poses in a magazine or book, those are better to be done "as needed".