Is it safe to use Cephalexin 500/twice a day against Acne for a period of 6 months. Should I use any Biotin at least?

Safe is relative. It is a utilized therapy for acne, as are several other oral antibiotics. But anytime you undertake long term antibiotic use, there is a chance of an adverse effect like antibiotic associated diarrhea, c.diff colitis, alterations in the natural bacteria of your body, resistant microbes. With that said, it's still rather safe.