get a copy from the TCS sensor library at my site
Maybe that was not clear, but it is bundled in my library for the TCS230 color sensor. Anyway, looks like someone else found it as well.
that this is needed for high frequencies (2kHz and up) i don't think that i will be in the need for such high frequencies
In the same place there is library called FreqMeasure that is for lower frequencies. How you count changes in order to retain accuracy.
Is this library neccesary for just counting pulses?
Not at all. You can just count pulses either by polling the I/O and detecting rising or falling edges, or you can use an interrupt to do the counting work in the background. The point is that your 'high' frequecy is actually not that high and the Arduino is more that capable of doing what you need (which answers your original question).
Your application is actually not a frequency count application but just a straightforward pulse count of the flowmeter for valve control, so use of the frequency counting libraries is probably not what you need anyway.