I'm not sure the reasoning behind the random sample, but if I needed to make sure that I'm only sending an alert for 25% of the population, anonymous or known, then I would try this:
If random sample = 75, remove from flow "this campaign". Default do nothing.
If anonymous is True, send alert. Default do nothing.
If anonymous is False, create task. Default do nothing.
Do some testing and see if that works out. You can also create another campaign as you suggested--up to you!