I think you have to take the SEL commercial in a single which is why a lot of AllATPs grads do not have it. It is a useful ticket because there are a lot of jobs out there flying singles for time building. It used to be the case you had to take the ride in a retractable, but I think they may have changed that rule (check). I would do as much in a single as you can, because even though twin time is valuable for logging purposes, there is also a lot more risk associated with busting checkrides in them. I recommend using singles as much as possible. Build multi hours later while teaching or doing block time packages in one.