red tails are primarily refered to as the Falcatus species.
I beleive altus is simular but maybe stays a tad smaller.
I have owned yellows,reds,altus, and 1 umknown sp and they all shoal togther as long as they are close in size.
My cuda's also shoal with hujeta's and SA needle fish.
If 1 of the fish in the shoal becomes less than around half the size of the biggest then it becomes shunned from the group and they will try to eat it.
I purchased 8 at one time and 2 died from fin rot, 2 died from a jump
And 1 ended up being a unkown species that grows slower than the other 3 yellow tails.
The remaining 4 shoaled very well up until the yellows got to 5" and the unknown was 3". I found it with bit marks all down its side. So I removed it.
After moving the yellows to a much bigger tank at 8" and months of power feeding the unknown. I reintroduced the little guy to the group at 4". They would not allow it to get close to them. So it stayed away for a few weeks and then I seen them chasing it so i removed it again. This time for good.
It will know live out its life with some charax gibbous and scomb payara.