Woah, you're going big. If you are brewing commercial, I'm positive it would be better to get a pitch from the labs directly sized for a 2BBL system. You would get a fresher pitch packaged in a way to ensure safe arrival.
The most accurate way to know your pitch rate is to use a microscope and slide designed for counting cells. A dye is also used to stain the dead cells in the sample.
There are methods to estimate the count including eyeballing and a dilution method that I haven't tried myself. I will use a yeast calculator, such as the one found at mrmalty.com, to estimate the number of cells in the slurry and eyeball it from there. Not perfect, but close enough for my needs.
Contamination can be avoided/reduced using proper sanitary practices. I simply wipe my fermenters opening with alcohol and pour into a sanitized jar. Not perfect, but works for me. We have a resident yeast expert that has discussed the best practices across many threads. His methods are akin to proper lab work, and I'm sure practiced by many breweries.