Hydration rucksacks on and off the bike? I suffer from diabetes insipidus, body fluids and inability to keep them inside.
I'm not a fan of using a rucksack on a ride, prefer the bike to carry the weight, but am wondering if a small hydration pack would be good for riding and walking when off the bike? Ideally I'd use some hydration mix, rather than just plain water. A good idea or would I be better with the bigger, 660 ml, water bottle and carry that in a handlebar bag over my shoulder when off the bike?
You don't say how long your rides are or how much water is needed per hour.  Going by my personal experience, you'll want 1 bike bottle (also known as a bidon) per hour in neutral/comfortable temperatures, and more if it gets hot.  

I did a 12 hour ride and ended up sucking down 8-9 litres of water over the period (a bottle and a 2 litre pack, filled up three times) and that is consistent with a bottle an hour.

Locations: You can have two bottles inside the main frame triangle. a couple on the bars in holders, and two more aft of your seat.  You can also put stuff in panniers or on a bike rack on the back.

Bottles are available in up to ~800 mL sizes too - don't bother with the little ones.   Don't bother with insulated ones either - they plain don't work at keeping the water cool.  Instead you can add ice cubes to both bottles add the hydration bladder before topping it off with water.

Liquid - hydration bladders are terribly hard to clean and dry, but if you don't they will go mouldy.   Only put water in a water bladder, as per the instructions.  Anything with sugar should not be in the hydration bladder.

If you want to carry electrolyte drinks, use a normal water bottle for them.   I've tried them but they taint the plastic, so I can still faintly taste the electrolyte powder flavour several rides later.

I tried using the hydration pack on my MTB frame, but there was a long way to lift the water up the hose to drink it.  I found the pack to be good on the back, and because it was 2/3 ice helped keep me cool for the first few hours of my trip.