According to the season 9 comics (issue #6) Nikki had Robin in 1973 right after she went through her Cruciamentum, so she was 18 at that time. Later it's said she had been a Slayer for 3 years at that point. Spike killed her in 1977, so she was a Slayer from 1970 to 1977 and died at age 22. So your head canon pretty much matches real canon on this one.
The 'Tales of the Slayers' COMICS puts her at MUCH older than 18, and there's no mention of Robin the story. It came out long before S9, which is where I originally got my head canon from. Like I said to [MENTION=6758]white avenger[/MENTION] in the other thread, I consider the story we're shown in 'Nikki Goes Down' to be the night of Robin's conception, because I find it hard to believe that a mother would spend so long away from her child for so long without mentioning him, if only to herself, or calling to check in on him, esecpially if Robin's so young.
But then again, there's also the 'Tales of the Slayer' NOVELS which shows Nikki going through her Crucimentium, and her Watcher finding out from one of her friends that she's pregnant and then worrying about the drugs he's been putting in her system.
The novel seems to line up more with what we saw in S9, but, like I said, I read the other comic first and that's what sticks in my head.
I think Joss confirmed that was the case, although I can't find where. It's why Halfrek recognized Spike in "Older and Far Away" and called him William.
I've had arguments with people saying that Cecily was the original human and that she didn't turn into Halfrek until after the events with William. Personally, I think this goes against the actual canon of the show, but then again, like I said, I'm not going to judge someone on what they think - even if I think it is in direct correlation to what we're shown/told.
Other head canons I have
I don't think Buffy and Spike had sex when she went down to the basement to be with him in 'Chosen'. I think they just slept.
I think River Tam is a Slayer but there is nothing left of the old organization to identify her as such, so she's left floundering on her own because she doesn't know what's wrong with her.
I believe that Dawn is Angel and Buffy's biological daughter, taken out of an alternate timeline in which Angel didn't have the day swallowed and Buffy got pregnant because they were both too lust-fueled to think about protection.
I don't think Buffy is that last Slayer we see being sucked into the portal at the start of the 'Fray' comic.
Anything that deals with other Slayers before Buffy, Kendra and Faith I consider to be canon. So the 'Tales' comics and novels are all canon to me, as well as 'The Book of Fours' and 'Pretty Maids'. (though I can't see to come up with a good reason as to why Spike doesn't remember killing Sophie, the Slayer, lol)
The Slayer Anna from 'The Lost Slayer' is the pink haired Potential we see getting ganked at the start of S7.
[MENTION=12223]EffyStonem[/MENTION] 's theory about Darla being a Slayer got me thinking about Drusilla. She had visions, could sense danger, knew that Darla and Angelus were vampires without anyone telling her. Slayer? It would also make sense as to why Dru went crazy when she was turned. Angelus tortured her, yes, but her madness far surpasses that. Dru being a Slayer would explain her madness, the soul of her Slayer self warring against the soul of the demon inside her.