Food is only used for 'creating' more population, money is only used for maintaining and making things. Morale is another factor in 'creating' (or stopping the loss of) population, and taxes are a way of having more creation capacity (by ways of money) in exchange for less population growth.
So, what about races that don't need money, or don't need food?
No-Money races: 'Industrial Capacity' represents the proportion of resources that are assigned deliberately or otherwise to the production of factories, ships, and prototypes-- the more that's put into these every week, the less that goes into the stockpile(BC reserves, money). To pay for more industrial capacity you may raise your taxes, and the lessened morale means slower population growth; why would this be so for a non-monetary race? Because less of their resources are being used to feed or create new citizens, and less of their resources are being used to maintain, feed, or refuel their citizens.
No-Food races: Everything needs fuel of some kind to keep going, if your species is immune to this then it's so powerful no other species in GC2 could compete with it. Fissionable material doesn't occur naturally, perhaps a race fueled by radioactive power cores needs a kind of 'farm' to find and process radioactives into a usable state. Without 'farms' of feedstock, most parasitic species would work through their food supply and be unable to grow their population or to continue to exist in a non-dormant state.
I think what it really comes down to is that if your species has infinite resources and therefore no need for an 'economy' of any kind, then it's too powerful to be a competitor in GC2. If your species has no need for food of any kind and can instead grow and maintain its population indefinitely, then it too is too powerful to compete.
