A flow regulator and a pressure regulator are entirely different things.
In general, a flow regulator consists of an adjustable constriction that limits the flow of a gas (or liquid, as the case may be) through an adjustable constriction. Usually a flow regulator requires a constant input pressure (unless it is preceded with a pressure regulator).
A pressure regulator essentially consists of a spring-loaded membrane that regulates the inflow on the high pressure end in order to maintain a constant pressure at the low pressure end, even if the input pressure varies (which it will).
CO2 in a cylinder is compressed, not liquid (as opposed to butane or propane). That means that the pressure at which the cylinder is filled will gradually diminish and eventually reaches ambient pressure as the gas is consumed.
The filling pressure of a CO2 cylinder can easily be 55 bar (about 800 psi) or more, while the pressure you need to dispense beer from a keg is typically around 2 bar (30 psi). Such pressure is not something to be taken lightly.
Don't try to economize or roll you own here. Simply get a CO2 pressure regulator for beer dispensing. Any other option is unsafe and must be advised against in the strongest possible terms.