An amino acid is any class of compound of the general chemical formula RCH(NH2)COOH. The term "amino" refers to the nitrogen component of that molecule. Only around 20 different amino acids are found in proteins in organisms - there are many amino acids that do not compose protein molecules.
WPI powders are derived from dairy and will contain the amino acids naturally found in proteins. However creatine does not form part of a protein molecule. Whilst it conforms to the general chemical formula of an amino acid, it is not a part of your standard WPI powder (unless of course it has been added in during processing).
We don't need creatine as an additional supplement, however creatine monohydrate has been well researched as an ergogenic aid for over 10 years. Personally I rank creatine monohydrate as one of my top 5 supplements. There are many other creatines on the market, but stick with your basic creatine monohydrate since every other creatine has next to no research to back it up.