It's my understanding that the 270 degree parallel twin mimics a 90 degree v twin in terms of firing order. I don't see how that would relate to any piston slap noise. The clackity clack is probably the valves. If they're a bit noisy, that's good.
To be honest, we have 8 motorcycles and I couldn't tell you if any of them do or don't sound "normal." I just put in earplugs and go ride. They all work fine and last a long time. I wouldn't worry about it. Go ride.