Is Zofran (ondansetron) ok to take occasionally with Prozac? I know Zofran (ondansetron) works via seratonin so wondering if it would mess with action of ssri

Usually OK. The medications won't lose their effect, but there is a risk of serotonin syndrome when you combine serotonin agents. Discontinue both medications and see your doctor, if you develop tremor, agitation, diarrhea, or seizures. You are not LIKELY to get those effects, but there is some small risk. Avoid tramadol (which is another serotonin drug) while on the combination.
Yes. Zofran (ondansetron) is actually an antagonist of serotonin: it opposes it. But it does so in a way that doesn't interfere with the effects of Prozac. (It antagonizes at the 5HT3 receptor, while Prozac works on other receptors.)
Yes. An interaction checker indicates there are no significant interactions between these two medications.