Thesp - Your guitar Centre oik is talking through his soundhole !!
String thickness depends entirelyon personal preference and guitar setup ! My D35 suits me fine with D'Addario J17's (13 thou first)
while my Cheapo Tanglwewood Earth is happiest with J16's (12 thou first) . Heavier guage strings on a decent banjo can result in a severe loss of tone , and both my 5 strings are happy with a 9 thou first (And Fifth)