I assume that the Imperial Credits are legal tender everywhere in the Imperium, and therefore there is no need to exchange money within the Imperium. The size and importance of the Imperium also means its currency tends to be accepted in nearby space, but the further you get away, the more likely to run into issues. Because of the wide differences in technologies, cash is used, but so are digital methods of exchange.
We know that prices aren't really fixed, because players make money off of prices going up and down - the speculative trade system assumes that prices are not fixed. The fixing of values in, for example, the CSC is just a convenience, so that we have a common reference for how much certain things should cost: prices can and should change due to circumstances.
Forbidding the issuance of other currencies is not necessary, and would be economically counterproductive. The Imperium consists of vastly different planetary economies and vastly different levels of development, and those with little of value to export will run out of Imperial credits pretty quickly without the ability to issue local currencies. This will be a particular problem for places that don't produce high tech items. Having a local currency allows the local government to increase the money supply. If you print money locally, you can prevent the local economy stopping due to a liquidity crisis, ensuring that local citizens have the money to complete transactions with each other - though they will not be able to buy imports. You end up with a two currency system: Imperial credits will be more highly valued because you can also get imports, but local currency is good for local goods and services. In some worlds, however, it is all just Imperial credits, because there are enough around and no reason to run a local currency. Local currency is hard to exchange off world - depending on the importance of the plant which issued it.
Most of the time I ignore this, but just bring it up if it matters for the plot. Characters use their credits and only when there is some issue around it, or when it is needed to show the PCs something about the locality they are in, do to we go into details.