Hadith are graded in authenticity as sahih, hassan, da'if and mawdu from the most authentic to the unreliable. But the Qu'ran itself outranks the Hadith, being the uncreated words of Allah Himself.
And in Q.5:48, we have
For each of you We have appointed a divine law and a way of life. Had Allah so willed He could have made you one people; but so that He ...
In the US it is common to say 'Happy Holidays' rather than Merry Christmas or Happy New Year. This practice became common because of the influence of the Jewish population, who like us, also cannot express wishes for festivals of others faiths such as Christian festivals.
It is not the first Question about talking/chatting. You find answers opposing each other here and here. The answers depend on how strictly Hadith is generalised, and whether a chat is considered to be "public". About personal physical contact only two persons of opposite sex, answer is quite clear, see here.