Am I allowed to put a wall standard within 18" of a fire sprinkler?
It depends on the shape of the spray and the rating of the sprinkler head. 

Sprinkler heads are rated for various situations. Based on the head rating, the sprinkler engineer can layout a pattern that “covers” the spaces based on the occupancy rating. That is to say, one kind of head will work in a narrow residential hallway, while another type is rated for business areas with tall shelving. 

Very complicated. I’d contact the sprinkler installer. Their name will be on a metal plate chained to the “riser” or where the water enters the building.