Is there a Talmudic mokor for a badchan specifically? (Grammer are common at weddings and shevah brochas-and I assume they are the same as a badchan)
More generall what does the Gemorrah say about humour as I tend to remember a story that a sage said that two people would have Oilam Habah as they made people laugh and presumably relieved their worries/stress? (Of course I could be remembering a chassic story rather than a Gemorah).
How is this different to a moishav leitzim?
The Gemara that alludes to humor is found in Ta'anis 25a (even though with regard to a Chasunah, we find the opposite, that we must remember the Churban and day of death at a wedding, as mentioned in Berachos 31a. However, it is also necessary to rejoice at a wedding, as we find with regard to Bar Kapara and Rebbi in Maseches Nedarim 51a).
Rejoicing, and allowing humor to lift one's soul and revive his joy, is different than being a Leitz. A Leitz mocks and belittles everything, especially that which involves growth in Avodas Hash-m. The genuine Badchan makes people happy and enjoy Divrei Torah and words of comfort.