Caution is always prudent when antibiotics and allergies are involved. Today's antibiotics are cataloged by families or types and by families of pathogens and mechanism for killing. Ciprofloxacin and Levofloxacin have a common molecular base called quinolones.
The risk of reacting to another quinolone is high enough to merit a warning by drug manufacturers. In fact Cipro comes from tetracycline varients and when Cipro was new, sensitivity to tetracycline drugs were in the warning. The "it's a completely different molecule" is not an assurance of anything although it's simpler than saying that most patients don't have allergic reactions to similar antibiotics.
A lot depends on how serious the reaction to Levaquin or such was. I reacted to my Shingles vaccination a few years ago. Within 3 minutes my voice went hoarse and I into full blown anaphylaxis, reversed with a pair of EpiPens. Normally flu shots are prepared the same way shingles vaccine is. I no longer get flu shots at a pharmacy.
If you are cautious, carry an EpiPen while taking antibiotics, trying a similar antibiotic may not be a choice. You know what the early signs of allergic reaction and without looking for trouble, be observant. It's a crap shoot that mostly works. If/when you become allergic to a drug, your options are limited. As pathogens develop resistance to antibiotics and we develop allergies to antibiotics only recycling older antibiotics or developing new antibiotics keeps the fight against infections.
LL
