What is a good antibiotic for cellulitis following the i. V. Antibiotic treatment?

It depends. There are bacteria that are resistant to certain antibiotics and are not effective. Your doctor needs to consider what antibiotics have been used in the past , possibly culture the infection to see what antibiotics will work now if needed.
Depends. There is no " one size fits all" unfortunately. Most common organisms are so called gram positives, so an antibiotic effective against staph/strep is most common- such as a penicillin or cephalosporine.G keflex, dicloxacillin. More and more, MRSA infections are the cause, so Bactrim (sulfamethoxazole and trimethoprim) or Clindamycin or Doxycycline might be better. Might depend on any underlying issues you have e.G diabetes etc.