Widest tyres on Shimano R500 wheels?

I have those wheels they were designed to use 23 to 28, so the only real question you have is will your bike accept a 28?  Only you can answer that one...or your bike shop.   Some bikes will not accept 28 at all regardless of fenders or not; some will be fine with a 28 on the rear but again without a fender, but will only accept a 25 on the front, almost all bikes will be fine with 25 all around, even though those may be fine at 25 they may not be fine with fenders.  Again this question is more about your bike and how it's equipped than about the rim.  

Those rims are very sturdy rims, I weight about 175 pounds and ride on mostly rough roads due to the freeze and thaw cycle breaks up the road really badly, in addition to that I ride outside the city limits and run into quite a bit of chip and seap roads; after 9,000 miles of riding I have yet to true them.  They may not be the lightest but they are bomb proof.

I have a Charge Juicer mid 2010 on R500's, with the supplied mudguards on you'll struggle to run anything larger than 23mm, remove them and you'll get 25mm on, anything larger might fit but will be a little risky due to the limited clearance.

You'll be fine with 28mm too.

Schwalbe Durano or Conti Gatorskins? If you've got the money Conti 4Season are supposed to be a terrific tyre. If you're not trying for a PB on your commutes consider Schwalbe Marathon. It's heavier and less responsive than the others but designed to cope with poor surfaces.

Best to go with 25 tyres,I've 2 bikes both with Shimano r500 and both with 25 tyres

You might get away with 700x28 if there's enough clearance under the frame/forks/brakes/mudguards. 25c tyres should be fine.