My newly made nightguard (not the hard plastic type) seems to fit ok but a bit too snug against my top teeth. can this shift or loosen my teeth?

Perhaps: If it was provided by a dentist i would make an appointment to have the guard adjusted. Perhaps the guard is fine and you are getting used to it. But a visit to the dentist is best. If you made this guard yourself it may be best to have it evaluated by a dentist. An ill-fitting guard may shift the teeth. I would say that loosening teeth is rare.

Adjustment needed: Call the dentist that made the nightguard and have this tight spot adjusted. It can make your tooth tender and sore and the tooth could move. The needed adjustment is common so call the dentist asap.