Yeah nen, the taller tubes are the best choice for Acacias. Sometimes they are called forestry tubes.
Shadow - you can grow things quite well in a pipe, I've seen it done quite successfully with Welwitschias a few times. My Welwitschias are ready to re-pot and I considered that method for growing them. Personally I wouldn't recommend it for an Acacia as they tend to have a shallow root system that likes to spread out just beneath the ground, but it will work if you manage it properly.
There's a few things you have to take into consideration, the fluid dynamics of the soil in a long tube are much different to the same soil in a conventional nursery pot. If you look at a standard nursery pot from almost anywhere in the world you will notice that they have very similar dimensions in regards to the height to diameter ratio. They have evolved that way because of the way soil/media holds water in a pot. A squat pot is generally not as tall as a conventional pot in relation to the pots' diameter.
The simplest way I can explain it is an analogy to a sponge like you'd use to wash a car. If you dip the sponge in water and without wringing it out sit it on its end so it's standing up (similar to a conventional pot) and then let it drain for a few minutes it will come to a point where no more water drains off and then wring out the water and measure it, it will hold certain amount of water (this is called hygroscopic water). Then if you take the same sponge, soak it in a bucket and then take it to let it drain on its side (which is comparable to a squat pot), when you wring it out and measure how much water it holds you will find it holds more water than it did when the sponge was vertical. So a short wide pot has a higher hygroscopic capacity than a tall narrow one That's why squat pots are used for plants that like wetter soil.
So when you scale the concept to a long narrow pipe you create conditions in the growing media where some weird things happen. The top half of the pipe will hold very little water because of the hygroscopic effect mentioned above and the bottom of the pipe tends to become overly saturated. So you need to keep a few things in mind when you're setting it up. The pipe must have very good drainage, I'd use a layer of rubble at least 2-3 inches deep (preferebly more in a pipe as long as you intend to use) and the pipe will need to have enough holes to allow air to circulate through the drainage layer, if that bottom section of the pipe stays wet it will become anaerobic & it could be a death sentence for an acacia. The top of the pipe will drain much faster than a normal pot so it will need careful attention (extra waterings) for several months until the tree is established. A soil moisture meter would be a cheap investment that will let you know what's going on with the soil.
