Ugh. Doesn't sound harmless (but there is not enough information here to say for sure). Wash the soil (and them) off or return to place of purchase. Don't worry about exposing the roots, just keep them moist. Here is a link that might be useful: The Myth of Fragile Roots

Many Sea Anemones live this way, and hold water within their bodies, squirting it out when squished. They often live intertidally, and "flower" when the tide comes in. One of the best places to find food for organisms that are fixed to the bottom happens to be rocks and other things in the zone between high and low tides. I would doubt that it is a Tunicate , or Sea Squirt, as they have two siphons, and this thing has only one pore. I have seen many of these on the Georgia coast.

They don't call them 'wrigglers' for naught! Here is a link that might be useful: Mosquito niiads in action!

Some sort of beetle larvae, I'd say, but don;t know what. I'll try to post the pic in the Oz gallery. Failing a proper ID, I'd just assume they were doing no harm as such in the leaf litter, and probably haven't invaded your compost heap, but if you were dubious just turn them out on the lawn and let the native birds have a picnic.