Sorry, I can not answer any of your questions.
The place you bought the medication should be able to answer your questions.
I buy injectable baytril from my vet and it is given orally. (Rats should never be injected with baytril.)
It has a very long shelf life (2 years), and is much more economical then buying oral baytril already mixed up.
Once a mediation is mixed up, it usually has a short shelf life.
I believe that some members on this forum buy baytril pills and mix them up.
Again, more economical and pills have a long shelf life.
You may want to investiagate these two options with your vet.
Personally, I only go through my vet (and pharmacy) for medications because I want to be sure of what I am getting and that it is safe and effective.
Edit: If you are going to be mixing up meds, you will need a good digital scale accurate to 1 gram, on which to weigh your rats.
You will also need to be able to do the math to calculate how much your ratties will need unless your vet is willing to do this for you.