Ok, so let's say you have a stable of four demons that you have defeated and enslaved:
Grimnos - A nasty little homunculus that looks like a naked old man with a bald head, bat-like ears, and a filthy tongue. He was the first demon you enslaved, with a Conviction score low enough (+0) that your apprentice level skills could best, but with the useful ability to throw bolts of fire from his hands as a Breath Weapon.
Mezz'Thun - A squat creature of living shadow, this demon speaks in a hollow voice and is as featureless as ink, except for twin points of lambent flames that serve as its eyes. Made of a bit sterner stuff than your imp (Conviction +1), it's still rather single minded, though useful for a Stunt that allows it to extrude tentacles of its substance and Grapple multiple targets, as well as its ability to take damage, since it has Armor 2 from Supernatural Toughness with a Catch of Holy attacks.
Belliona - A seductive demoness who approached you about making a pact. You're still not sure as to her reasons, but you have definitely benefited from the relationship. If it weren't for her tail and horns she'd be a strikingly beautiful woman of seemingly Middle Eastern descent. She uses her Human Guise, a Superb Rapport score, and her Incite Emotion (Lust) to good effect against your enemies and for the entertainment of your friends. Quite a bit more willful than your first two demons, Belliona has Good (+3) Conviction.
Thul'Zan - A minor noble in the demon hierarchy, Thul'Zan gave you the fight of your life when you summoned him. Manifesting as a hulking humanoid with plates of spiked armor fused into its skin and a greatsword the size of an I-beam, this demon's Great (+4) Conviction meant that you were moments away from being decapitated and eaten when you managed to beat him by the skin of your mental teeth. Grudgingly he knelt and swore fealty to you. This is your heavy hitter with Inhuman Speed, Supernatural Toughness, and Supernatural Strength.
I'm going to assume that the Containment and Binding parts are already done and just concentrate on the Summoning spell. According to YS273, when summoning an entity, the complexity of the spell has to beat the creature's Conviction roll, suggesting a complexity of 10 to be a good round number to beat the best roll of something with Superb (+5) Conviction.
According to YS288, using Sponsored Magic to summon quickly basically works like Evocation, you summon shifts of Power and then roll Discipline to control the spell. You take one hit of +1 stress for summoning the power and then one hit of (Final Spell Complexity - (Conviction + Complexity Bonus*)) stress. The Discipline roll is made at the Difficulty of the total shifts of power summoned.
* I'm allowing +Thaumaturgical Complexity to factor in here instead of +Evocation Power because I think your Faustian Bargain bonus should count when summoning demons.
To be able to summon Grimnos (Conviction +0) reliably, for example, you're going to want to summon at least 5 shifts of power to beat a possible roll of +4 on his part. Then you'll need to roll a 5 or more with Discipline to control the spell. Let's say you have a Conviction +5 and Discipline +5, which I'd highly recommend. Then the default specialization bonus you get from having Thaumaturgy (+1 Diabolism Complexity, let's say) as well as the bonuses you get from Faustian Bargain (+1 Complexity, +1 Control) give you an effective Conviction +7 and Discipline +6. You can easily summon the power and control to get you your imp, you can even take it as a Rote, if you want, meaning that you can decide to take an automatic +0 roll and forgo the possibility of getting a negative roll.
To summon Thul'Zan (Conviction +4), on the other hand, you'd want to summon 9 shifts of power. This means that you would take a 3-stress hit, 1 automatic plus 2 for the difference between 9 and your effective Conviction of 7. Then you'd need to meet or beat 9 on a Discipline roll. Better pump some Fate Points into this. You could also take Refinements to get more Focus Item slots and increase your effective Conviction and Discipline scores for the purposes of summoning your demons. Each Refinement gives you +2 to your Focus Items. Take 3 Refinements and you can pump yourself up to an effective Conviction +9 and Discipline +10. With those kinds of scores, you can be summoning even Conviction +5 demons with a Rote.
All of this, of course, depends on the Conviction scores of the demons you want to summon. This requires adjudication by your GM as to how much Conviction a particular demon should have.
