The key here is proper training and exercise. The GSP is going to be a world apart from your mastiff. I've owned mastiffs and compared to sporting dogs they're just lumbering hulks that prefer to lay in the middle of the busiest part of your house - becoming an almost immovable obstacle to work around.
Sporting breeds WILL be VERY energetic (even English breed labs will be energetic compared to a mastiff) and that might cause a problem between your mastiff who already lives in the house and a new puppy. Be aware of the situation and don't leave the two together unsupervised unless you're ok with a crushed puppy.
As I said before most people don't know how to properly train a dog and that goes double for sporting breeds. If you're looking at obedience classes be sure to have in mind a set in stone list of criteria that you want. Do not compromise on this list. I'll give you my list as an example. Yours may or may not be similar to mine.
1. Dogs MUST be submissive to people.
2. Dogs are not allowed to jump up on people.
3. Dogs are not allowed to lick people.
4. Dogs are not allowed on furniture - that includes the bed.
5. My yard isn't fenced in or any of my property for that matter. Dogs must know and mind their boundaries.
6. Dogs must not rush out the door when the door is opened - rather they must sit and wait to be commanded outside.
7. Dogs must not rush into the house when the door is opened - rather they must sit and wait to be commanded inside.
8. Dogs may not eat till they are told its ok to eat.
9. Dogs will sit on command EVERY time no matter the proximity of their master when the order is given. This is synonymous to "stay" since they should remain sitting till ordered to do something else.
10. Dogs must return to their master when the order to return is given regardless of the situation or distance they are from their master.
11. Dogs must cease barking upon being commanded to stop from their master.
12. Dogs must obey their master regardless of the presence of other people, dogs, animals and/or sources of distraction.
These dozen requirements of mine are mine and when I train dogs for someone else I train their dogs to these requirements. If they choose to relax a bit on the discipline and say let their dog onto furniture that's their choice but its always easier to relax the discipline than to try and tighten it after a dog has been allowed to do something on a regular basis.
When looking at a trainer make sure to look at their personal dogs and decide if their dogs behave the way YOU want YOUR dogs to behave. Don't just judge them by how other people's dogs are but judge them by what you want out of your dogs and determine if the trainer is capable of doing that. I don't bother going with a trainer's previous customer reviews because they will almost always get rave reviews. This is because most people haven't got a clue what to expect out of a dog much less one that has been well trained.
Also don't just go by the diplomas and/or degrees they might have on their walls either. Some of the best dog trainers I know don't have any of these things - just showcases full of ribbons and titles from tests, trials and shows. There is no actual licensing requirement (at least on the federal level and not in my state) to be a dog trainer. I do believe it is that way in most states too. Anyway those diplomas and certificates don't mean anything except that the trainer has spent the money to get those. What matters is results. Look at their personal dogs, their clients' dogs and their accomplishments in the past to get a good gauge on their quality of training.
That said I'd strongly suggest finding a local hunt trainer in your area for your GSP if you get him/her. The reason for this is that while you may not bird hunt the hunt training is a good foundation for other types of hunting (ie. small game - rabbit, squirrel, raccoon) and the obedience that is trained into the dog through hunt training is so far above what the average non-hunting dog trainer would give. Get a properly trained hunting dog and you'll never think that dogs you'd previously thought were good dogs are all that good again.
If you get that puppy good luck and enjoy your time with it. Have fun with the housebreaking lol!
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