Hi!
There are wormlike slimy predators that eat earth worms. They can really
reduce worm numbers. Also tiger worms don't thrive in the cold!
Alan
you said:-
snip------
Something in the back of my mind says that composting worms (eg
tigers) cannot survive in garden soil, and earthworms cannot survive
in compost unless there is a certain amount of soil to go through
their gut.

I just throw everything I can find, and a bit of soil into cut off
corrugated iron tanks, and I seem to have plenty of both sorts in
there - but I have never found a tiger in the actual garden.