When I was buying a Plecos I’ve read that they do not grow more that 14sm??? And 20 inches is about 50sm (is it correct???) O my God… of course if it will grow that big my tank is very small….
When you were buying, do you mean going by the info that the pet/fish shop gave you? Usually they highly under estimate the size of the fish that they sell.
-I am using a strap at the moment, but will buy a liquid test as you recommended.
The best liquid test kit to try and get hold of is the API Test kit, you can buy the entire test kit (ammonia, nitrite, nitrate, high pH, pH) or you can buy each individually and they last for ages
-How do I know if it is the bacterial infection, I mean how can I recognize it?
The first is to look at the symptoms, behaviour and physical appearance of the fish, sometimes it is quite difficult to judge. You will also need to test your tap water to make sure that it isn't just stress of fluctuating water parameters that is causing them to be off colour.
If the tap water GH, KH and pH are different from the tank what should I do?
I wouldn't think about this until you have tested your tap water and have the results. There are a number of ways to change the water chemistry, I would leave it to someone that is better explaining to post up

Just stay away from any of the 'pH up' and 'pH down' products that you see in the pet stores, they are just plain useless and inflict more problems than they are worth
-Guys tell me – DO I HAVE TO FEED THEM at the moment or not??? Poor fishes they are coming to the front glass as soon as I’m sitting and watching them … I do have some live plant in the aquarium and have noticed that some of them nip/eat it. At the moment water in the tank is very clean, and I can not see any white spots on any fish…..(the only fish which had it was Loaches and a Knife- but they are not there any more). All the fish are very active including the Plecos (they are swimming around, Plecos are on the sides or under the wood or in the live plants…. )
I would feed them but keep a close eye on your water parameters and try and get a reading for ammonia as soon as possible. Might be best to throw in a few extra water changes a week just until you know your readings. Have you tried some cucumber/courgette in with your plec's? All you have to do if remove the seeds and slice in half so the fish can get to the flesh, blanch it in some hot water for 5-50mins, pop it on a fork so it sinks and leave it in for an hr or so, they will love it
The thing with whitespot, is that it can stay hidden within the gills of the fish for a fair while, you can look at your fish and it could be weeks before it physically shows up on your fish, this is why you must keep up with an ich treatment even after all physical signs disappear, it can be stubborn at the best of times to get rid of
Ashleigh