It is not necessary to put tetras in, and I would advise against. Anytime need additions are made, it is possible to introduce pathogens your precious discus aren't immune to. The QT process is slow but necessary to protect your investment. I would (did myself) just start with the discus.
I would not use muck water or media from an LFS as that could introduce pathogens into your tank which may make your discus sick or even kill them.
My advice is to slow down, cycle your tank properly with a fish less cycle using ammonia as your starter. You will need a complete test kit to monitor your water & cycle progress. Once you have a properly cycled filter (0 ammonia, 0 nitrite, and the presence of nitrates) add your discus.
Rushing it, not doing due diligence and not doing a proper start up cycle is the main reason many new comers to discus have a disastrous experience...that and adding discus from different sources or to an existing tank of other species without the proper quarantine measures.
I'm not trying to make life difficult for you. Discus really are amazing fish. I went through all of this when I got mine about 10 weeks ago. They are happy and healthy and I am glad I took the time & effort to do it right. You will be too!