My brooder-raised chicks tend to start roosting on their own at 10 to 12 weeks of age. I've had some start as early as 5-1/2 weeks, some take longer, but 10 to 12 weeks is a good average. This is in the grow-out coop where there are no adults. If they are in the main coop with the adults it's different. My broody-raised chicks are different. I've had a broody take her chicks to the roosts at 2 weeks, though 3 or 4 weeks is more normal.
My broods are typically around 20 chicks. I didn't notice if you said how many you have. I have not had any issues with them suffocating by piling. They sleep in a group on the coop floor but they don't really pile. Suffocating by piling is generally when large broods panic at some disturbance and pile against a wall trying to get away. With living animals anything is possible but suffocating while sleeping in a group is not on my radar, I just don't worry about it.
As long as mine are not sleeping in a nest and are sleeping somewhere that is predator safe I don't care where they sleep.
Roosting is when they sleep up on something at night. Perching is where they play on something up high during the day. Two different things. I've seen them perch on the roosts during the day weeks before they started sleeping up there at night. I have a small perch for them to play on in the brooder, but they typically only play on that during the day. They don't sleep on it at night but I usually move them to my grow-out coop at 5 weeks. I have not noticed that having a play perch in the brooder making any difference when they start roosting at night.
If it is that important to you that they roost up there, move a few up there when it is too dark for them to fly back down. Dark is a key. You do not need to move them all up there, if one or two start to roost the rest usually soon follow. If you try this try to move the same ones up every night. Consistency really helps with this.
