for scales i like to take a tongue depressor and cut the end to form a point with curved lines leading to it. When i depress this into the clay it forms uniform scales. What you could do is lay some cotton inside your appliance and coat it with the liquid latex. When it's tacky, use the tongue depressor and press it in to create your scales. This will give it a realistic 3d look.
If you're going to an all day event, I highly recommend a product called "sweat stop" by michael davy. It will help prevent your perspiration from loostening the pros aide. I was volunteering in a haunted house last year and the full face appliance i was using started to come off at the mouth and nose very quickly. I had to do a lot of touchups but since the skin was already oily and sweaty, it didnt work out very well. Had I been using sweat stop it wouldnt have been a problem. Even though my face was clean and I prepped it, the sweat buildup gradually loostened the grip that the pros aide had. Fortunately it had a great hold on most of the major areas (cheekbones, bridge of the nose, brow etc) that my appliance never looked unrealistic but it just became uncomfortable to wear as it was no longer moving with my face.
If you're using sprit gum just make sure to clean your face thoroughly to remove any oils that will interfere with the adhesive. the same goes for pros aide. I can't say enough good things about pros aide. I've used it on appliances that I've made for the neck and other areas of the body and it will not come unstuck without a solvent.
