Halichores chrysus
According to Scott Michael the get to be about 4.7" and I'd be inclined to agree but I have seen one about 6" long once, in a 600 g tank.
I've kept two. One I had about 14 years ago, he jumped and went floor surfing, we saw him and put him back in the tank. Couple of months later he went AWOL and we thought he was a kitty snack, but found him in the SUMP about 6 months later
Put him back in the main tank where he lived the rest of his days. I can't remember how long we had him but it was a long time.
The last one I had I got in early August 2000. Bought him MO and he arrived with ich, missing scales and his tail was all torn. Nursed him back to health, and all was well til a few months ago, he appeared to develop an internal parasite. Treated that and he was on the mend, but something else got him. I think his immune system was weakened and something else got the better of him. Had him only about 13 months.
GREAT fish..they sleep in the sand, they are inclined to jump but there are an animated and colourful addition.
As for sexing, not sure, but I know that I've read about some going "super male" and developing nice green stripes around the head area. I've read about this but have not seen it for myself.
I think they will not bother your gobies. I've kept mine with seahorses (14 years ago...not recently I "know" better now! Had no probs though), clownfish, a P. annularis, coral cats (look a bit like engineer gobies), he didn't even bother feather duster worms, however they are a risk with ornamental shrimps, snails and fanworms...so use good judgement.
Most recently I've housed one with Coral Beauty, clowns, citron goby, pyjama cardinal and 6 lined wrasse.
HTH
Jenn