BB Gobies are not brackish they are freshwater. Trust me did lots of research.
10 Things to Know About Gobies: Bumblebee Gobies, by Peter McKane
Mine are VERY active and healthy. Regardless I use a tad of aquarium salt in the tanks anyway for other reasons.
Im going by what I have seen in the stores, what they told me and what I have seen myself and read.
If food is readily available and they dont feel the need to compete you should be fine and they will not harass each other. Not saying one could just not be a jerk in general tho.
They are no where NEAR as messy as others say, Maybe Im lucky or maybe its cause I do a 50% wc every week.
Its hard to know male to female because they
1) one will not show until maturity by a line under the belly and some by the spots
2) I have also read these fish like others almost all start female and then change as they mature.
Anyone who says you are getting a pair is prob either not 100% guaranteeing it or sending you older more mature fish and if thats the case like any animal they will be set in there way and may not take kindly to others. Mine both came from the same tank in the LFS with lots of java moss and I put them in a tank with lots of plants and space.
At first I start second guessing my purchase as they NEVER came out and it was impossible to feed them (except snails) but they soon mellowed out and are all over and never bother anything else nor each other and the gobies do great.
On a side note I accidentally sucked one BB Goby up with my siphon like 2 or 3 weeks ago. Poor little fish almost went down the toilet but lucky some strange psycic link made me look in the bucket first and the poor guy was start white and confused. I NEVER saw a fish swim himself into a net to be caught.