I don't get the point of nice'ing "cd" at all (because that's what you are doing). '&&' is a shell control operator and it is catch by your shell, it is not interpreted by nice (in fact, it doesn't even reach nice at all, in case that was your intention). Maybe you meant this instead:
cd foo && nice -n -18 ./srcds_run <whatever_else>
nice sh -c 'cd /foo && ./srcds_run <whatever_else>
But, as said, niceing cd is completely irrelevant, it's an instantaneous command that just runs and gets you into a directory, it's not like it'll be running for two hours or wasting cpu time at all.....