To me, I think all FM is almost the same. Of cos, when U start trying, always mix it to see if it works for your child before switching it completely. I don't foresee any issue to start stage 2 FM at ard 10 mths old. For some FM in the past, only got stages 1 and 2 and of cos, people will stick to stage 2 until the kid is totally wean off from FM unlike now, there are so many stages available.
I read that this is quite normal. Stage 2 formula would contain more calcium and iron to meet baby's growing needs as well as his next stage of development and this, unfortunately, can lead to hard stools or even constipation. If baby is already eating solids on top of the formula milk, perhaps you can include more fibre in his diet as this can help loosen the stool a wee bit?
Sure. why not? Just ensure the age is reached before trying out the different stages. To start stage 2 at a later time is acceptable to me too. All these stages are just indicative only.