There is nothing wrong with SMZ pills. It is just they are not a very strong or powerful antibiotic -- more suitable for a cut leg that you don't want to get infected on an older horse.
Foals can get so sick so quickly, that I have settled on a Vet that treats them very aggressively. He will tell you that he did not previously treat them as aggressively as he does now. He took more of a 'wait and see' approach and then treated conservatively only going to more powerful antibiotics when it was very evident that they were needed. After losing foals that he should have saved, he changed his approach. Suits me just fine because I, too, have lost foals that I do not think I would lose now.
The other thing wrong with SMZs is that Sulfa has been around over 60 years, so really serious infections are all resistant to it.
Gentocin and Amakacin are specific for 'gram negative' bacterial infections. These are really difficult infections (like Pseudomanas and Klebsiella infections) and these antibiotics are much more effective than Sulfa or Penicillin that are more specific to gram positive Streptococcus infections.
Hope this makes sense.
I hope your baby is not very sick and only has a little local infection. It is better to be aware of what can go wrong thatn wonder what did when it is too late.