On filing, it depends on whether you’re doing an inlaid design or a 3-D design. For an inlaid nail, I will file the acrylic to a smooth finish then apply a UV-protectant sealer.
(You can also use a clear gel or acrylic.)
With a 3-D design, I smooth and seal the underlying nail, but there is no need to file the 3-D portion since I use my brush to form the design itself. I also omit sealant on the 3-D design. As to whether you can mix and match monomer brands, it depends on what brand of colored powder you are using. Some colored powders can be used with any company’s monomer, but others recommend against mixing and matching. Just check with the manufacturer and follow their recommendations.
— Elaine Watson