I dunno bout the harleguin with bigger fish but I would think he'd be ok. As for catchin him and if he never comes out, taking your tank apart would probly be the only way to get him out. Or you could try luring him out with a piece of raw table shrimp and tongs. My serpent always comes out when I feed him but my brittle is shy and never comes out just like your harlequin. It'd be a little easier to get my brittle out if I needed to since my tank is only a 30
30 L aquarium, 50 lbs LR, 44 lbs LS, 192 W PC, CPR Backpack Skimmer