No cypro works for syphilis, however I would recheck after as I have heard of people still testing positive and needing something stronger. The do the weak stuff so less resistant strains form.
Penicillin indeed is the treatment of choice, in fact the only recommended treatment. Cefalexin has never been studied against syphilis. It probably would have some activity, but cannot be considered reliable treatment -- and it could screw up further diagnostic testing, for example to judge whether it had actually worked.
If you would like to provide more information about your syphilis diagnosis (stage, blood test results, etc) I might have additional thoughts.
I'm not sure what ng33 means by "cypro", but ciprofloxacin (Cipro) is totally inactive against syphilis, and is not related to cefalexin.