I don't build lugged frames only fillet-brazed but I was taught that silver brazing is the preferred method for lugged frames. The tolerances are supposed to be pretty tight and the brass won't flow as freely as silver. With that said, I know that some builders do use brass with lugs. As far as flux goes I haven't used Harris flux but I would imagine you always need to flux your joints to prevent contamination. If you are pretty good with the silver brazing why not stick with what works, the temps needed are much lower and less stressful on the tubes. Good luck.
Dimitri