The injection of terramycin and the spectam both are an antibiotic to help prevent these diseases.
You should really do one or the other, because if they got both, and then had disease later on in life, anything in their system has been exposed to two different drugs, and may be harder to treat.
As such, I would go with giving the Spectam and see how it pans out, that way you have the Terramycin if any turn ill later.
Sorry to hear about the little one, but it sounds like it was for the best, she tried, but with more than one thing to fight (legs and watery mouth disease), it must have just been too much for her, and you gave her peace.
Sometimes these problems come along to give you something to learn from, and I think you're absolutely right, making sure that they have something preventative like spectam and plenty colostrum will help the others yet to come into the world.
Hope the rest don't give you as much trouble
Suzanne