When to take OG, when pitching on a yeast cake?

I normally take my OG before I pitch my starter? I am pitching onto a yeast cake today, what is the procedure.

Fursther more what is the order of putting wort in the carboy? 2 gallons of water, then wort, then top of? I usually start with 50F water, will this temp have any adverse affect on the yeast?

You would rather chill the yeasties a bit instead of heating them up with your warmer wort. But if your wort is already down in the 80's range then it really will not make any difference which way you do it. If it is any higher than 80, then you need to put the cold water in first. As for pitching onto the cake, rack as normal.

You can take the OG after you've pitched and topped up. Fermentation won't start THAT fast.

Not sure if 50F water would shock the yeast. If I were you, I'd pre-mix some of the hot wort and some of the 2-gallons of water to get it in the 70-80F range (maybe a gallon total) and pitch that first. Then the rest of your 2-gal water, then the hot wort, then the final top-up. That should minimize the temp extremes for the yeast.

One more question, what about aerating the beer? Do I shake the sh*t out of it once everything is in the carboy? Or whould I just wisk the boiled wort once it's hit 80F in the pots?

Shake it all. The water in the carboy will hold some O2 as well.

Or, you can aerate that separately, I suppose.

2nd beer is fermenting, but thedeposit at the bottom of the carboy is almost an inch thick. Should I swirl the stuff once in a while to get it in suspension.