I think summoning should be largely based on the amount of chakra you have, since that is the main component on performing the justu. (I.e a lvl 10 cant summon gamabunta, but could summon small toads because Bunta has a chakra requirement of 500 CP) As for how to obtain the summons. Meh, there are many ways to do this. Quest chains through Npcs, or like how some people have said just fight, beat, then interact with an animal. Summoning is an exciting part of the universe and I'm glad its being added, but making it work is going to be very complicated I think. But Im interested to see how it goes