In a 20 or larger they make a very nice active species tank. I got 5 pairs on line and put them alone in a 20H. I planted it heavily with easy plants like hornwort and java fern. End result is a very active tank that is filled with movement and color. Lots of fry survive with the heavy planting and they are incredible little guys.
I find that Endlers are more interesting in a species only tank or with other livebearers such as platies or mollies, maybe some shrimp on the bottom to help with cleanup. And they do well in a tall tank, as they will occupy all levels.
You can certianly keep them with other very small fish such as tetras or white cloud minnows but I found that Endlers and White Clouds (in my case) did not compliment each other aesthetically They just do not seem to belong together in the same tank.
I kept my Endlers with a couple of Bumblebee Gobies for a while. Kind of interesting combination. The BBGs occasionaly picked off an Endler fry. Kept things lively.
In a 10 you could easily have 15 or 20. I would have to watch the 55 to decide when enough is really too much but for the health of the fish I would stop at 75 to 100. They are very small and pose much less of a bioload than most aquarium fish, even the little neons.
Note that they are prolific. If there is adequate cover, 4 or 5 pairs will fill the 10 or the 55 in less than 6 months. The next question you will face is where you can get rid of lots of young endlers. They are very active and fun to keep but I have never seen one at the LFS so don't expect there to be much of a local market for me. I am almost at the point where I will need to remove some fry cover or start trying to get them into the LFS and find out what the market is like.
Violet gobies are brackish, and could easily fit an endler in their mouthes. I take it it's in the 55g. I would put either a bunch of males or 1 male 2 females in the 10g.
my violet goby i keep in FW. ill put the endlers in the 10 get a steady population going then trans some over to the 55. the violet goby should keep the poputlation under control