Here's a bit of advice about training - keep the dogs separate while you're working with them.
If you're working with the GSP keep your mastiff inside/locked in a different room etc. Don't let them interact while you're training or else your training will not be very fruitful. Likewise when you work with your mastiff.
Having multiple dogs and training them at once can be a handful and even for experienced trainers can be a big obstacle. That's when you have dogs of the same temperament and energy levels. Dogs of such different temperaments and energy levels can be even more difficult.
Don't buy a puppy unless you can see a verified pedigree (3 generations or more) and medical proof that the parents aren't carrying any genetic defects and/or diseases that will come up in the puppy as he/she grows. I'm not an expert on what afflicts GSP but in labs you want proof that they're clear of things like EIC, CNM and PRA and hip/elbow displasia. Also definitely get a health guarantee - 2 years is typically acceptable. This is always a must too when you're paying anything more than a couple hundred dollars for a puppy.
In regards to the pedigree make sure that the pedigree shows plenty of documentable and trackable hunting titles (such as MH, AFC, FC etc) and the like... or whatever titles are used in pointing breeds (as they are somewhat different from flushing retrievers). If you're paying more than a couple hundred dollars for a puppy you'll want at least some titles within the grandparents and at least a SH title in both parents. If you're paying more than $500 you'll want some champion titles (AFC, FC) in grandparents and/or parents and definitely look for MH titles in one or both parents. Anything close to $1000 or more and you'll definitely want lots of championship titles and MH titles throughout the pedigree. Basically you'll want lots of MH titles in parents, grandparents and great grandparents on both sides. Also a bunch of AFC and FC and perhaps Master National Champion in the parents and grandparents. In the hunting field this is quantifiable proof that the puppy has good genetics and is capable of accomplishing what you ask of him/her. Even with all of this you're taking a gamble... just less of a gamble than if you bought a puppy with no pedigree or just a plain jane pedigree.
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