Sharktocrab's triggered ability can target a creature that's already tapped. That creature won't untap during its controller's next untap step.

An ability that triggers when counters are put on a permanent will trigger if that permanent somehow enters the battlefield with those counters.

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.

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