I haven't done much with metal studs, but I've built a couple of boxes in a similar style to what you want. I've used regular angle profiles. It can be a bit tricky to get the screws to drill into the metal. The easiest way is actually to pre-drill with 1.5 - 2.5 mm straight through the gypsum and metal.
But you must fireproof the pipe, especially against the combustible ceiling.