As long as the pleco is large enough for the aro not to attempt eating it on impulse, it's fine. How big is the tank? Common plecos or plecos that are as long as 12" often fit perfectly fine with adult silver aros.
There is a photo on indodragon - aro swallowing a pleco ... the OP did not reply yet whether the meal was completed and whether the aro survived ordeal
in a 2 1/2' tank the arowana will be stunted before its big enough to eat the pleco- and in 2 or 3 months when those fish are gone, make sure before you get an alligater gar or arapima to replace the aro, check what they eat before you put it in your tank