Can I alter the developer mixture to control the contrast of a print?
Yes, to a limited extent – much less than is the case with film.
No but, stop bath is cheap and avoids problems from developer carrying over into fixer. Staining and reticulation are prevented, and the life of the (more expensive) fixer is extended.
One liter of QUICK SILVER concentrate will make 10 liters of working solution, capable of processing (600) 8×10 fiber prints or (900) 8×10 resin-coated prints.
You probably don’t want to do this… In a print, you usually want some good, solid, maximum black. Most print developers are designed to be energetic enough to give this in a couple of minutes, as long as you expose the paper correctly. By contrast, when developing film, you want to avoid maximum black as this makes a negative that is very hard to print. So film developers are much less energetic than film developers, and when used with paper, give weak, muddy, prints. if you have to, maybe try doubling the concentration… raising the temperature… agitating continuously and vigorously.