Is it considered bad form to have a novel where the majority of it is 3rd person limited, then change to omniscient for a few scenes? I think I can build up more tension if the protagonist is not in a few scenes.
Like I wrote on another answer about POV, don't overthink it.
There are writing rules, but rules are meant to be broken (just broke the avoid clichés rule). Make sure you do it at appropriate points in the story (a chapter break is probably best). Make it easily recognizable to the reader that they are no longer in the protagonist's head.
In the end, if switching POV confuses the reader and pulls them out of the scene, causes confusion, etc., don't do it. If it helps move the plot forward, provides necessary intrigue or suspense, etc. etc. Go for it!
Get a beta reader if you're not sure. Write it how you think it works best. If the beta reader notices the switch and expresses confusion or disruption because of the switch, then you know it doesn't work.