YAY! Another betta lover!

35 liter is a great size, too.
As for adding a few other fish, bettas are solitary and s/he will prefer your wife's company to that of other fish. Their nature is aggressive, and they don't get along with many other fish. Two exceptions are corydora catfish or otocinclus catfish for algae eating. You can read some of the posts in the betta-specific section of the forum; not everyone is in agreement as to this, but many who have tried have ended up with maimed or dead fish.
I'd do as Rbacchiega says, give the tank a week. But be sure to feed the bacteria though! I don't think a week without
ammonia would increase their number.