from what i have read yes you can add it mid cycle it is then a little less than instant to cycle your tank
but it has to be better than having fish die on you
that sucks as mine all died
so it doesnt make you feel too great as a new fish tank owner
i paid just under 20 pounds including postage but have ordered extra to be safer
financially has to be more economic than needing to replace dead fish
or to medicate fish that are sick due to going through the cycle
everyone says a qt tank is a good idea especially if you have the room for one
but again you will need to keep it cycled somehow, i think advice is to keep some filter media in your tank or to use some of the media from your display tank when using the qt tank
someone who knows better than me can step in here with more information
good luck hope that you manage to stabilise your tank one way or another,
before you run into any more problems