Is the hadeeth of 72 sects correct? I have heard that the sanad of the hadeeth is not sahih.

The hadeeth is authentic and there are many different narrations of it. The group which is going to Paradise or firqa najia is not ‘hair line’, for example, if only your toe is out, you will get thrown out – not like that. It accepts differences and it does not mean everybody has to agree to everything. The Sahaba and the tabi’oon differed. The fiqh or understanding may differ.