From the FCI standards:
...they are especially appreciated as family dogs due to their calm, reliable temperament.
&
BEHAVIOUR /TEMPERAMENT : Self-confident, alert, watchful and fearless in everyday situations. Good-natured and devoted towards people familiar to him. Self-assured with strangers. Medium temperament.
and re behaviour with other dogs - never encountered a problem with any of the swiss mountain dogs I met when walking my dogs (mostly encountered bernese but the greater swiss is more or less a short coat bernese).