In general, the acceptability of any specific corrosion rate depends on the design or expected life of the corroding part.
Most standard corrosion tables consider corrosion rates lower than 20 mils per year (0.5 mmpy) "acceptable" on the premise that most corroding components have been designed with a logical corrosion allowance that will provide the desired service life. However, in the pharmaceutical or food business the acceptable rate may be very much lower because of tainting/contamination of the product.
If you put corrosion coupons in cooling water stream you should have a purpose - evaluating the effects of water chemistry changes on the materials of construction, or trying to define the corrosion conditions so that life prediction can be made, or determining the corrosion in the worst locations so inspection frequencies can be devised, etc. It is essential to match the materials properly - nothing is learned about copper tubes from carbon steel coupons, and so on.
When you know the purpose, come back on here and see if the test you're planning makes sense.
One test is worth a thousand opinions, but only if the test makes sense in the first place!!