I think the answer to your question is fairly simple; you can train most dogs to do most tricks if you try for long enough and of course if you and the trainer have the desire to see it through. Gundog training starts at birth and takes years to complete, I don’t actually think any dog is 100% there until its 5 years old (that will cause some comments)
Most professional gundog trainers will be into running dogs in field trials or training to work with the gun, they will have fairly set procedures and will expect working stock puppies which depending on breeding have a good desire to hunt and retrieve. Their job is easier that way. My gold dog has so much working instinct its almost easy, she may even show me how to shoot soon.
A spaniel is a dog that was bread to do the whole shooting job so will keenly retrieve hunt and Flush, although people think that workers are bigger this is not always the case, I have seen the smallest cocker bitch carrying the largest Cock bird with no problem, not quite sure how she could see where she was going though.
My own cockers will retrieve or hunt, they will stand peg all day, go for a days beating or have a walk up day through some of the worst cover in the world, they go anywhere. I have even shot geese with my cockers.
If you really want to go shooting with the dog get some one to one training, it’s the only efficient way to get the best out of the dog, start with an assessment with a good trainer who will be able to tell you what kind of chances you have, if you are just looking to pass certificates have a look for a good training book or video, Simon Tyers sells a very good Video.
One last comment, in reply to the sexist comment against men the mix of males to females working dogs in shoots and competition where I go is about 60/40, if I think of just cockers there are probably more ladies out there entering competitions or working on shoots.
Us shooting lot are nothing to be scared of, most of us will try to help anyone.
