The fish store wants you to cycle with damsel because they want to sell u a fish. Dont do it, it is cruel to the fish and it could die. Also damsel are very territorial and you would have a hard time adding something without it getting picked on, and they are nearly impossible to catch without taking all your rock out.
Cycling with a fish has no benefit. Raw shrimp is best method in my opinion, or just dosing household ammonia.
29g Mixed Reef, 150w 14k Phoenix MH w/ Moonlights, Reef Octopus BH1000 HOB Skimmer, Ecotech Vortech MP10es, ReefKeeper Lite (Plus), PH Probe, Temp Probe, ATO, Carbon/GFO Reactor.