Guru Gobind singh dealt and was friends with many, So he did liase/respect all including his enemies (remember the europenas were lurking round the corner preparing to take over india-so there was all sorts of people around in India at this time) and expected the same back (e.g. Zafarnamah a letter to the Mughal Emperor at the time). So he would not turn someone away for having cut hair.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sikh-youth/8997-is-it-true.html
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=8997

Forgiven in the past for having cut hair: of course sikhism would, ther is nothing really to forgive. The hair is a path towards Gur Sikhi, it doesn't condem any other person or religon for doing any different. Before the Visakh of 1699 many sikhs had cut hair (see Dr Gopal singhs writings) , it was after this date (due to people denying being sikh in fear of death from the mughals) that the hair was a requirment.