How do summoning jutsus work? Let's have a back drop on the story, the Third Hokage shows Jiraya, Tsunade, and Orochimaru his summoning technique. This intrigues Jiraya and he too tries to perform the jutsu without making a blood seal with an animal. As a result he is teleported to Mount Myoboku, where the Toads live. He trains there for some time. 

Here lies my first question, until later into his training Jiraya had not made a blood seal with the toads, so how did he get to mount Myoboku to undergo his trainings? The reverse kuchiyose should only work when there is a blood seal with the animal. Going to Mount Myoboku on foot is a long and tiresome journey, so this possibility too is cancelled out. So, in order to go to Mound Myoboku did he use the kuchiyose no jutsu every time? 

My other question is what decides which animal a shinobi is to bond with? There is a wide range of creatures that one can choose from, but certainly there must be some sort of compatibility to be worried about. What made Jiraya decide a toad summon would be best for Naruto? 

Has anything that can substantiate the above points been mentioned in the manga?
Nope, it isn't explained at all.

The background you described is 100% filler and was never described in the manga, so using the Summoning Jutsu without a blood pact doesn't necessarily teleport you randomly.

As for which animal is suitable for which. Jiraiya seemingly decided to give Naruto the frog contract because he had the contract himself, (not to mention that Naruto's father, Minato, also had a blood contract with frogs).

I don't think there's a requirement of any sort, but that's just guessing. The entire technique is cloaked in mystery.

See my answer on this other question: How to summon a different animal using Kuchiyose?