UKC

My lovely wife has got me a hydration vest with bladder for my birthday for longer runs 

https://www.amazon.co.uk/AONIJIE-Hydration-Reservoir-Marathoner-Backpack/dp...

The instructions say that you should only use water in the bladder. I usually run with tailwind in my bottles. Anyone ever put this or squash in their bladder? 

I have done that. Obviously, you need to rinse it out and keep clean.

Some bladders are hard to dry out. A good trick is to store the bladder in the freezer when not in use. That way any leftover humidity won't grow mold. 

I've used lots of energy powders in Camelbak and Platypus bladders without issue.

Unless there is something specific about yours, the only issue I can think of is one of hygiene. Perhaps the manufacturer has taken a look at the biological warfare breeding ground that is the inside the typical rehydration bladder drinking tubes and wants to have no responsibility for the mutant life forms that might emerge from one of these someday.

I have done so in Source, Platypus, Camelbak and Decathlon no-brand bladders down the years. It can get gungy if you leave juice or squash in there with out washing it, but rinse it out and store it in the freezer seems to prevent any mould. I did invest in a cheap cleaning kit off Amazon - same stuff as you get in the Camelbak or Osprey ones, but a third of the price https://www.amazon.co.uk/Hydration-Bladder-Cleaning-Universal-Reservoir/dp/... and that makes cleaning it much easier. Might be worth buying if you think you're going to use it a lot.

They'll get funky whatever you do. I generally sterilise them with Milton or something similar, then flatten and put in the freezer until the next time I want to use them.