And how do you propose he does that?
This sounds like ditching more than half of the miners using this pool...
Not at all. If too many coins of a single currency are sold at the same time there comes an oversupply, thus the price can go down quickly. The point of balancing over many currencies is to reduce the chance of oversupply of any single currency at any point in time.
This can be done, for example, by finding out how quickly a currency drops once middlecoin starts mining it to the point when middlecoin has sold all the currency. It also needs to be done with the same currency without middlecoin mining it to find out the effect middlecoin has.
There's more steps that need to be done. Its just educated trial and error to develop this but its pretty straight forward to do. Its basic statistical analysis
To answer your question specifically the pool will load balance the different currencies being mined