by the way, my purple tang does tail slap him every once in awhile but he is not bothered in his little corner on the back wall...
If the tang doesn't let him out of a corner, that will be an issue.
For now, can you catch the yln and keep it in a quarantine tank until it eats and settles in? Say 4-6 weeks. A 10 gallon tank with heater and HOB filter or powerhead can suffice. Just change the water frequently to manage water quality.
the only mysis i use is oregon desert mysis and brine shrimp, its a local company around here and the quality i think is better then PE...