In a 36-gallon tank, the betta crowntail may have more space to avoid the serpae tetras, but it is still not an ideal pairing. Introducing new fish to the tank may further stress the betta crowntail and potentially make the situation worse.
If you are considering keeping these two fish together, it is important to carefully monitor their behavior and be prepared to separate them if necessary. You may also want to consider adding some plants or other decorations to the tank to provide the betta crowntail with hiding spots.