Right now he (she?) is in a 135, before long I'll be moving him to a 185 that is 4' square. They aren't terribly active, but they are very inflexible. This makes it important that they have a wide tank as they get larger.
Nigers are very inoffensive towards other fish. Mine is about 12" and lives with a 1.5" Hillstream Loach and 1" Tetras.
It made me a bit sad to see that big one in the photo Mystus Redtail posted, that's how big my bigger one was when he kicked the bucket. I bought two of them, both about 4" long. After a few years (maybe 3 -4?) they had grown to 18" and 24". I don't know why one grew so much more than the other, they were fed and housed the same.
Yes it was sad to see in person too. That's why I put my gf in for a size reference, he was truly a monster. There's no doubt that he couldn't turn around in there, just go back and forth. Too bad he wouldn't fit in my carry-on...