The fuel mix is something that is the same in real F1. It's not cheating at all just like a previous poster has said you will use up more fuel during the spell when it is use. You can also use it during the race but be warned you will run out of fuel of you use it too long.
I am guessing the the issue of the Williams not having the different fuel mix available is an R&D thing. I think you will find that during the season if you complete your R&D objectives at certain races the new fuel mix will become available. It is available to the bigger teams from the start of the season.. a result of a bigger budget but the smaller teams have to work on it during the season. It was similar in 2010 with things like the engine mapping, etc.