I believe starting their graduation into Genin Teams at age 14 would be fine, if we consider by "timeskip" they would be roughly 16 or so.
I don't mind starting with them as young as six or so, though if we did change the ages they join the Academy, we'd have to take that into account. If they graduate at age 14 and started at age six, that's about eight years in school when I had assumed it would be only three, perhaps four at the most. So they would probably have to be ten or so at the start of the roleplay for a few posts back and forth for the sake of introductions and first impressions.
I don't mind OCs coming in as temporary characters, though to apply one to a Team when there's already several canons we could add for drama (and due to any history they share with Shikamaru or Sakura) seems like a choice I'd avoid. The idea that they'd both be trained under a shinobi is an interesting idea, though it see it perhaps happening later on if say both people make in into the finals of the Exams, then I can see them training together (if they did both make it to the finals, they could even be facing one another but that is a ways down the road).
Since it could be hard to figure out what team we'd want, I'll try to outline my general ideas behind some of them.
Sakura, Sasuke and Naruto & Shikamaru, Choji and Ino (Keeping it canon) - Shikamaru and Sakura could still attempt to meet up outside of missions while dealing with their feelings toward certain teammates. There's the Naruto likes Sakura who like Sasuke issue. And the Shikamaru and Choji are bffs with Ino kind of flirting with a few shinobi for fun. (though IMO, she never really had a thing for Shikamaru, it depends if you'd want to fiddle with that)
Sakura, Sasuke and Shikamaru - Sakura would still be dealing with her crush on Sasuke while trying to maintain a friendly relationship with Shikamaru. Might slowly see the contrasts in personalities between the guys.
Sakura, Naruto and Shikamaru - Naruto and Shikamaru and Sakura would probably get along fairly well as a team. I'm sure Naruto would make some quips about how amazing Sakura is which might help any sort of "crushes" develop. Of course this is the beginning teenage awkward feeling stage so nothing is set in stone.
Sakura, Ino and Shikamaru - This probably has the biggest chance for drama if we did play up Sakura and Ino's rivalry and her (feelings?) thoughts on Shikamaru. Then there'd be the constant back and forth about Sasuke being so cool...I think Shikamaru would kill himself. Hmm.
Shikamaru, Naruto and Sasuke & Sakura, Ino and Choji - A way to the boys all together, same reasoning applies to the canon idea. Naruto and Sasuke could both be voices in Shikamaru's ear. One enjoys Sakura, the other is clearly indifferent while Ino and Sakura deal with their own drama along with poor Choji.
Shikamaru, Choji and Sakura & Ino, Naruto and Sasuke - Just swap Ino for Sakura who probably would cling to Sasuke like glue and pretty much be indifferent about the loud "other" blond in the group while I could see Shikamaru, Choji and Sakura having an okay dynamic.
As far as any romantics are concerned, crushes can be developed, fade away, come back. I do intend for Sakura to get over her thing with Sasuke during the timeskip era. Or to at least accept that she and Sasuke are just friends at best, though this may be hurried if she and Sasuke aren't on the same team which could then diminish her little crush on him. Of course if neither Sakura nor Ino were put with Sasuke, that might help even more since I'm sure if Ino got Sasuke, she wouldn't let Sakura forget about it. If Shikamaru and Sakura do develop "something" around the Exams or after Naruto and or Sasuke leave Konoha, it's possible it would depend on certain circumstances and if something did happen, there's no reason for it to turn into something huge.
After all, they could easily try dating and then go back to focusing on training or something, the idea could still be lingering in the back of their heads.
There is the classic (cliche ish) slice of life situation at Christmas or a party where they could be pushed together and have some kind of innocent kiss. Though since they're young, I'd expect to see some cutesy kind interactions anyway. And you did mention some various ways to get drama with Naruto or Sasuke. I think Naruto or Sasuke could be nice plot devices to use. ^^