I'd say about 10 stone max, but not that much for jumping or hard work. I used to have a 13.2hh NF pony who I kept until I was 21 - I used to ride him (weighed about 9st at the time) and he coped fine. It depends on how well balanced the rider is as well. Allow less weight for an unbalanced rider.
It depends on the horse and how short coupled they are. The longer the back generally the weaker it will be. My YO is probably around 10.5 stone and the NF we have bought in to sell struggles with her weight. But then it is very weak
He's a 7 year old I've been helping to break. I've grown really attached to him but I weigh 10.7 stone and am worried that I will be to heavy for him when he is in proper work. I'm quite balanced and not looking to do any serious competing - just hacking and some local dressage and jumping. He's quite a solid chap really and hasn't seemed to struggle in the slightest with me so far - if anything he seems to need reminding that he has a rider at some points!
He will carry you no worry - NF ponies can carry a stone a hand - that is how their weight carrying ability is traditionally worked out and its still deemed as accurate!!
I think that would be fine. Just make extra sure your saddle is a good fit as if it didn't fit well a bit more weight would have a greater impact than if he was ridden by a very lightweight rider
I have a 13.1 section b - she is out on loan at the mo but I used to ride her and I am 5ft 2 and 9.5 stone - I rode her in a 17.5 inch saddle - quite big for a little pony but she was very long in the back and the big saddle distributed my weight over her evenly - my saddler came out and checked it - she wasnt stocky either and we bombed about everywhere!
I think you would have a problem riding with a height more that weight - as the pony above was quite short in the neck and had a low head carridge and this felt scary when galloping as I felt that there was nothing in front of me! but as long as you feel happy and the pony is going well - you should be fine - they tend to stagger for a few steps if you are too heavy for them as they get used to the extra load.
11 or 12 stone ish - does spend on the rider's ability too as if you think about it, a novice will sit more heavily that someone who been riding for years