Can you get panniers on a pelago commuter (large) without the additional pannier support thing they sell?
Yes. 20L 'rear' Ortlieb panniers for example work just fine. Securing the panniers at the bottom was no problem at all with my Ortliebs.
The panniers are a little high up and forward, so not in an ideal position. But this never really bothered me. Toured with this setup for years.