Do you have to follow the comics?

It's up to you if you want to or not.

If you haven't read all the comics or any of them, that's completely okay.

Like I said, how you choose to interpret Buffy's happy ending is up to you.

If Buffy's happy ending is after Chosen, that's perfectly okay. If you want to fudge with the comics or any of the series generally, you can totally do that.

Do you have to follow the series?

Again, you don't necessarily have to follow the series although Buffy generally didn't have much in the way of happy endings during the show.