I'd watch what goby you get, I have an engineer goby in my 125 that's a good 7 inches long. In terms of the cardinals, I heard they can be aggressive (not 100 percent sure) I'd personally go with a clown pair, just make sure they are 2 different sizes if possible, this will cut down on the fighting to establish dominance. Oh and watch out for the maroon and yellow, they tend to be territorial. In terms of a ph, I'd get one just for good measure, if you're running a slightly oversized hob
filter it'll prob do the job, but it wouldn't hurt to have a small ph.