Adding a Sub-Panel to 3-Phase Main
Yeah, that plan seems fine. Three #1Al THHN wires should get the job done, and will fit comfortably in 1-1/4" EMT pipe.  You don't need a ground wire in EMT, IMC or rigid conduit. 

You sometimes hear of neutrals being downsized compared to the hots.  You  can't generally do that with feeders, but especially, you can't do it when pulling 2 of 3 phases of 3-phase "wye".  Here, neutral does not handle only differential current, and it gets a real workout - neutral flow can be as much as the hots. (but not more).