I'm explaining my logic wrong again... I can never get my words to come out right the first time...
Ok... here'summoning.
Just like any series of Jutsus, you have to start with the basics. Summoning is no different. When first signing a contract, with the right oamount of chakra, you can summon grunts of whatever tribe you focus on.
Generally, beginners can only summon smaller creatures. These creatures (if they aren't babies) are generally children of their kind. These creatures are generally useless in a fight, depending on the tribe... For example, Toad Summoners, upon first signing, may summon minor frogs like Gamakichi or Gamatatsu. Far too young to be useful for anything aside from messengers. As they gain experience, and as word of the Summoner reaches the Boss via his underlings, the Summoner may try to summon the Boss... and upon impressing the Boss or making arrangements with him, the user may gain the ability to summon higher tier toads.
Now.. if the user has the chakra for it, bnothing stops the user from summoning higher tier toads BEFORE meeting the boss... the only problem would be whether or not the summon will obey. For a Yakuza-like clan like the Toads, weaker members can do as they will, and are how Gamabunta judges the character of the summoner... but even if they sing his praises, the middle and higher tier toads will most likely not listen unless Gamabunta says so.
Of course, there may be differences from Tribe to Tribe, but essentially that's the way i see it...
Now... upon gaining tyhe Boss' ok, either through a ceremony, an oath, a ritual or whatever they may require... the Summoner may learn or have access to jutsu that are only available to contractors. A stipulation to this may be that the ninja who learns these techniques are forbidden from teaching them to anyone else save for other contractors to the Tribe.
For example... a Toad Contractor like Jiraiya can learn, and then teach the Kichiyose: Frog Stomach techniques to Naruto, since he signed the Toad Contract as well... but he couldn't teach it to Anko, because she is not Contracted to the Toads.
Orochimaru can teach Sen'ei Jashu to Anko, but not to Kidoumaru for the same reasons.
Of course.. this only applies to major Summon Tribes. Major Tribes have Bosses, and require far more song and dance to learn techniques and gain power from...
Minor Contracts, like Kakashi's dogs for example, don't really have a Boss. They only grant minor benefits, but expect good treatment. Instead of the large variety of beings liek the Major Tribes have, the Minor Tribes are relatively small. Again, using Kakashi's dogs as an example, the amount of dogs he summoned in Wave against Zabuza is the extent of their tribe.
Now... what possible reason would someone sign a Minor Contract over a Major one? Familiarity. Since the Minor Tribe is small, one can get to know each tribe member, know their strengths and weaknesses, and thus easily summon them for whatever circumstances you might need them for. Meanwhile, with a Major Contract, this is far harder to do without a guide. Only those within the Tribe would possibly know of a member who can help out in needed situations... and even then, it would be difficult to summon that one creature unless you accidentally summoned them before hand. Unless you automatically force a large amount of Chakra into the technique to call the Boss, the entire summoning situation for Major Tribes tends to be a random grab bag.
Of course... this is where special summoning techniques and special scrolls come in.
Once you find a Summon that works for a situation, the Contractor can make a minor contract with that being in the form of a special technique. Examples of this can be found in Jiraiya's 'Food Cart Destroyer' and Orochimaru's 'Sen'ei Jashu'. For the very first summon in a needed situation, the Summoner most likely thinks 'I need somethign for such and such situation'. For Jiraiya's case, something large enough to crush objects from above without relying on Gamabunta. The result? His first time summoning the big toad, and from there, a creation of a minor contract to allow him to summon that same toad for use in a technique. In cases like this, familiarity helps.
As for Jiraiya's Toad Stomach Shield/capture techniques... well.. Familiarty is needed here as well. After all.. not just ANY Toad would allow you to summon their stomach for situations. And even then, most likely a tribute is needed to be made to those who hold minor contracts.
.... I think I covered all my bases here... but I'm not sure.. I'm tired adn school is gonna start in a couple days ;-;