If you have a corner grounded delta and have a two pole breaker that lands on phase B, it must be straight rated (240) and not slash rated (120/240).
This is due to the fact that a ground fault on phase A or phase C is 120 volt, but a ground fault on phase B is 208, which is in excess of the individual pole rating of the slash rated breaker. See 240.85
To answer the second part of the question, no, not all 2 pole breakers are straight rated. In fact, most are not. Last time I saw, a straight rated breaker costes about 5 times as much as a slash rated.
