Can an Unseen Servant trigger an Alarm spell, and can said Alarm be customized?

As an Unseen Servant is "shapeless", its size is not defined. The Alarm spell, however, relies on a creature’s size (Tiny or larger) to be activated. Therefore, I’m not sure if an Unseen Servant, when commanded to walk through a location containing an Alarm, would trigger said Alarm or not. I would think it does, but I might be wrong (maybe the Unseen Servant’s size value is "null"), hence the question.

Also, one of the two ways to receive an Alarm notification is mental. I’m wondering whether that notification can be customized, so that if the Unseen Servant crosses through Alarm #1, you can get a different "ping" than for Alarm #2. Again, I would think so, but it’s better to verify (maybe it’s always the same sound regardless).

Finally, I’m almost certain that if you go back and forth through an Alarm zone, you will repetitively get the "ping", but I just want to be 100% sure.