I keep reading that they need/prefer low flow rates in the tank, so why is it that in my tank they all fight over which one can swim against the flow of the filter tube?

There is one that does it more than the rest but he won't share, it's his spot and the only time you don't see him there is feeding time..

Interesting - they're only youngsters aren't they (quite small)? I found all my baby Angels loved to play in the flow from the waterfall HOB filter I used in their grow out tank and the three I kept went crazy playing in the bubbles when I added a bubblewall to their tank last week. My adults though, tend to hover in the quieter areas of their tank most of the time, but even they will occasionally pop to the more turbulent end for a quick "shower" in the spraybar output.

I think it's size related: when they get bigger, they find it harder to balance and manoeuvre in a higher flow so are less inclined to want to do it, but juveniles seem to make a game of it

Yeah, it's the biggest one that does it most, looks to be about 2 inches including fins. Any time he sees another one there he chases it away, he even attacks any of the terra that go near it. As soon as they flee, he just swims into the flow.

They're still small then, around the size of my three juveniles. The biggest of those is about 2 inches (excluding fins) and the others about 1 3/4 and 1 1/2 and they all do it.

The parents are a lot bigger 4 1/2 inches long and 4 inches tall excluding fins; including fins they are around 5 1/2 long and 6 1/2 tall. They move more slowly than the youngsters and spend a lot less time in the high flow areas.

Sounds like a typical case of kids playing then ☺