I think that a 55 gallon tank for be a luxery suite for the 2 of them, but a 40 or 45 would work. And I don't have any experience with rectangular triggers, but they are closely related to assassis, bursas and picassos, so I guessing that they aren't that aggressive, but they can be. If you can a rectangular trigger, snowflake eel, and a sharpnose puffer, I would say at least a 90 gallon tank or so. But I don't think that they would work well together. Triggers and puffers don't work especially since the sharpnose is pretty small it would be easy prey for the trigger.
-Mark