If a creature somehow loses all of its +1/+1 counters, it can adapt again and get more +1/+1 counters.

Pteramander's activated ability can't reduce itself to less than to activate.

You can always activate an ability that will cause a creature to adapt. As that ability resolves, if the creature has a +1/+1 counter on it for any reason, you simply won't put any +1/+1 counters on it.