if you are using old water because you think its gonna cylce the tank, you're wrong.
most of the beneficial bacteria is on the surfaces of your tank, especially your filter media, and not in your water column itself. if thats what you're going for you should take filter media out of your other tank and put it in your new filter.
if you dont have enough biofiltration in your tank and filter then your tank is going to cycle and you're gonna have problems
as far as hardness is concerned, what are you putting in there that needs soft water? are you trying to breed? if not your fish should be able to adapt just fine, just do a drip acclimation.