Hello Colosseo and welcome to fk

sadly a lot of fish shop staff are not in the know when it comes to fishless cycling a tank, or they are told what to advise, and a lot are old school where you use fish to cycle it and end up like yourself with dead fish

Cardinal tetras are quite sensitive at the best of times so not good for a un-cycled tank
To answer your question, you can start to add the Ammonia straight away

we do have information on this method so will find it for you

another method is using filter media from a mature tanks filter not the water, so do you know anyone local that has a mature fish tank set up at all

here you go
http://www.fishkeeping.co.uk/articles ... hless-cycling-article.htm