You cannot use straight tap water. You must put declorenator in the water to remove cholrine and other chemicals, that the treatment plant puts in. Those chemicals will kill fish if not removed.
Many people would say that you need to put in water that is the exact same temperature, I just make sure that the water is within a few degrees of what I keep my tank at.
On part of adding fish. It is suggested that you add the hardyest fish first, and only a few at a time so the tank is not overwhelmed by the new bio-load.
It is best to have patience, ten fish is way to many to add to a new tank at once. Cardinal tetras have a reputation of being a not so hardy fish, I would add the Plec first and hold off getting 1/3 the Cardinals for at least two days. Then a few days later again get another third then get the rest a few more days after that.
I would suggest a drip aclimitation, a quick Google could tell you all the pros and cons of different types of aclimitation.
I will touch on this subject again, because I have failed in it so many times. Fish keeping is not fast. We can't accomplish anything over night. Fish keeping is about making small changes and carefully observing to reach a final goal. Just because you go to a store to buy fish does not mean you should walk out with fish every time. If something seems fishy (pun intended) like the condition of other fish in the tank, what the employee is telling you or anything, it is usually best to be safe than sorry. I have easily wasted $300+ by not being patient enough. I hope my failer can be your saving grace.