Copyright law varies in different countries but basically what you ask in your first question would be a breach of copyright.
I know people use magazine photos all the time, but it's still a breach if you want to sell, exhibit or publish the work.
If you ask the photographer's permission to use the work, and say for what purpose, it may well be given - but you need to ask!
You may use the photo for 'fair use' and educational purposes - in other words, for practice drawing or painting the pose ... but not for selling afterwards!
As to your second question - there are many 'free figure poses' around - or drawing cds/videos where you might find the pose you need - then you are free to do what you wish!
It's often said that if you copy, but change say 60% of the objects/elements in a painting, is that OK? .... personally, I'd say 'No!
I wouldn't a) like my work taken and changed in that way and
b) wouldn't want to steal another's work in that way.
So now it's your choice!
Forgot to add - if you do use something for practice and post it here, don't forget to credit the photographer - preferably by name too, not just e.g.'Thanks to the RIL'!