There are three cases where a stand & shoot is not allowed:
1) Enemy is too close
2) Enemy is out of the front arc; or
3) The weapon itself is "slow to fire"
Since the giant blowpipes don't have the slow to fire rule, only the other two conditions apply and steggie can S&S so long as the enemy is in the front arc and not too close. However, since the giant blowpipes suffer all appropriate penalties, they are likely to require 7's to hit since the enemy is almost guaranteed to be beyond 9" (i.e. -1 for multiple shot, -1 for range and -1 for S&S) and thus no poison will apply.