Using water from your established tank achieves little to nothing in regard to BB transfer. Using 50% of your substrate from your established tank, as well as a filter, will put you on the right track, especially if the new arrival is small. In the meantime, adding your other temporary occupant, just to give you an ammonia source, is also a good shout.
However, my concern is not for your new tank, but for your already established tank, the one where you've took 50% of the substrate out, as well as a filter. That is a lot of BB you've transferred and if you've got fish in that established tank you need to keep your eye on your parameters and/or monitor your fish for unusual behaviour.