Are all merit sources have same smerit generation?
No, I believe each merit source have differing number of sMerit generation per month. There are some merit sources who generate less sMerits while there are others who generate more sMerits. Only Theymos and those merit sources themselves know that how many sMerits they get per month.
Another question is once a merit source can be a lifetime merit source? Or is it also pick and drop system like DT¹
No, merit sources are different from DT members because a merit source can stay at his/her position as long as he/she is helping others with those sMerits, he/she is active on this forum, and he/she doesn't break the rules of the forum.
There have been some merit sources in past who aren't merit sources anymore. If I'm not wrong then a few months ago Ratimov was a merit source too but now he isn't a merit source or reputed member of this forum. Similarly, there were a few other merit sources who left the forum and aren't merit sources anymore.