Yea basically there's no hard set rule about it, but it's not out of the realm of possibility that some families would, especially after living with other races so long, want to emulate the 'family names' customs by setting up their kids' names so they have matchy last names. It's entirely on the family, though, I don't think any Lalafell would think you strange to do either. Personally for me none of Zutoto's family has a shared last name, but she'd find it very cute if someone did that for their kids.
