Oh don't worry, hydrotech's are staying on bogo for a few more days (Nick said so himself), but not sure if it will remain for the whole 2 weeks.
And definitely the 3 hydrotech's are enough. I ONLY use hydrotech's (and I have the tiny 3" ones too, THEY'RE SO SMALL!!) and I get phenomenol results even with heavy compounding
BTW the hydrotech's on BOGO are actually 5.5", so you've got it good.
It should; yes you'd exert less pressure overall with a thicker pad but not to the point where you can't do any corrections; it still works nicely (in canada I only have access to the 1.25 anyways and it works flawlessly)
Yes they will work but I prefer the 7/8'ths so I didn't jump on the BOGO. And IMO LC Flat pads are some of the best "Bang for buck" pads. You have to change out the Hydrotech pads much more often then the LC Flats.