Is it safe to take medications like Cialis or viagra (sildenafil) with a penis implant?

Penile prosthesis. Its not that it's unsafe, but more so that it doesn't make sense. A penile prosthesis completely replaces the erectile chambers. There is very little to be gained by taking pde-5 inhibitors after having a penile prosthesis implanted. If the prosthesis is not creating full erections, see your urologist. There may be a problem with the device.
Usually. What kind of prothesis? Malleable or inflatable? Assuming you don't have any other contraindications to a pde inhibitor (cialis, viagra, Levitra (vardenafil) or certain medical conditions) you could take them but usually the prosthetic is put in because of failure to have an effect with medication.