OK, if you have a regular submersible well seal, it should have three holes in the top. One is the 1.25" hole you can use for the hydrant. Another would be a 3/4" hole that is usually used for the wire, and a third hole is probably ½" with a plastic plug. You can put a screened vent in either of the other two holes. Just use two elbows so the screen faces down and rain can't get in. You won't hear any hissing, without a vent, the air and contamination will draw in from below ground level.
Any water that goes back into a well can contaminate the water. If the 1.25" and other holes have gaskets, it will keep out the rain water. However, when you shut off the hydrant, everything in the hydrant and in any attached garden hose will go back down the well. I would remove the hose before shutting off the hydrant. The water in the hose could contaminate the well, and it could start a siphon that would draw a puddle in the yard, or the water in the dog bowl, back down the well.
That type of tank doesn't even hold ½ of a gallon. It also has a tendency to loose the air charge. A small leak in the house can cause a lot of cycling, and if the air charge is lost, it will click rapidly on and off until the pump is destroyed. You can add an additional pressure tank anywhere in the system. Even a 4 gallon tank that holds 1 gallon of water, sitting on top of the water heater or under a sink would help.