Ilove congrats on the new tank! Sounds like a good find. You can also use 1 part bleach to 20 parts water for cleaning and sterilizing. Make sure to rinse everything until you don't smell the bleach. Then you can fill the tank and add a double dose of
chlorine remover let it set a few mintues, then start with fresh water just to be safe (also with chlorine removed of course. )
I can't say exactly how long it will take to cycle the tank using the substrate from an established tank but it won't take as long to cycle compared to if you didn't use it and started from scratch. Too, if you can spare any filter media that would help also. However, don't take away so much good bio matter from the established tank to throw it into a mini cycle. Keep an eye on your readings in both tanks.
Another tip...You could add the new filter media into the 12g tank for seeding prior to setting up the 55g. Leave it in there for at least 2 weeks. This will give you pretty much an instant cycle. (if you have room in the 12g that is)
Best of luck and keep us posted.
Ken