Whatever happens, do not raise your voice at customers. Mistakes happen so if an employee does find themselves shouting at a customer, they should find the time to apologise to the customer, whether they were right or wrong... shouting is unacceptable. Either the employee should apologize, or the manager on duty.
Checking that nail lengths are equal on both hands should be standard protocol.
Usually I love this place but I found the staff quite rude on my last visit. Furthermore it was a very rushed visit and it has only been 5 days and two of the gels on my nails have heavily chipped and are peeling off. I'm not happy with this, nail varnish would have lasted longer.