Fill the beer up to the 'weld line' of the kegs.
You'll probably be fine regarding not getting starsan in the posts, but do it next time.
Never heard of any issues with starsan/co2, I do it all the time and I highly recommend it.
1 - Clean keg with napisan to your liking.
2 - Put 1L of Starsan/Water mixture in keg and slosh it around right way up, upside down and roll it around so that starsan touches all inner surfaces of the keg.
3 - Hold keg upside down and pull on pressure relief valve so that some starsan comes out of it. Then close it.
4 - While still upside down push in the gas poppet so that some starsan comes out of it.
5 - Put it right way up and optionally allow it to settle for a while if you
fear the foam. I don't bother.
6 - Hook gas up to gas disconnect, purge keg a bit to get rid of some oxygen
7 - Hook beer disconnect up with a length of hose going into a jug to catch the starsan, or if you have a bronco tap setup use that. Let the co2 force the starsan mix out as if it was beer. It'll start just pushing foam out after a while, that's fine, but if it starts pushing gas out with the foam stop it. You can wait a bit for the foam to settle and push more out if you want, I don't bother.
8 - Disconnect the beer and gas disconnects from the keg (it should still have co2 at pressure inside it)
9 - Position it under your fermenter and hook up one of these
http://www.craftbrewer.com.au/shop/details.asp?PID=1118 to a liquid disconnect. Push the liquid disconnect over your beer out post on the keg and probably more foam will come out. Position the end so that the foam goes into your closed fermenter tap to sanitise it. Once the gas is only down to a hiss push the silicone hosing over the fermenter tap. You now have sanitised co2 filled keg and tubing from fermenter to keg. Nice.
10 - Pull the pressure relief valve on the keg so that it's stuck open.
11 - Open the fermenter tap. The beer will go down the tubing, down the kegs beer dip tube and into the bottom of the keg. You don't need to worry about splashing because the keg is full of co2. Once the keg is near full the starsan foam that is now sitting on top of the beer will start coming out the pressure relief valve, giving it a bit of further sanitation and letting you know the jobs almost done. Once full to your liking disconnect the liquid disconnect from the keg and close the pressure relief valve.
12 - Burp the headspace out of the keg and carb the keg in your desired way.
Additionally you can use the tube linked to above to fill a bottle or two at a pinch if you have leftover beer in the fermenter.
I learned this from argon and it's some of the best advice I've ever had here, and that product above is the best $7 I've ever spent on brew gear.
