I'm gonna go out on a limb and say that combat summoning isn't different. Just hard. You're compressing the timeline but it's still at least three rounds. At least. Here's why...you're summoning a trump card. Which you can totally DO! That's magic! But! The bigger the trump the harder it is to pull off. Need a demon in a hurry? You better negotiate hard and fast! Or you end up having to give it your Name. Or your Soul. Don't have time to do it right? Well we all know what happens when you go cheap on a summoning, or get distracted. KABOOM!
I think this is totally doable as one of the higher-risk/higher-reward kind of deals. Just make every step take at least 1 round (so 3 rounds minimum) and just up all the difficulty. Item quality, time, distractions, all are BIG factors in a summon. So make'em count. Does that make on the fly summoning near impossible? Yep. But that's something to build towards. Summoning isn't JUST about calling them up. It's about control. So your summoner, like the most powerful wizards, is REALLY SCARY when they get the chance to prepare. So if they hat up, and do a proper summon, then great! You brought a boomstick! But if they do the summon, and you aren't ready to USE the critter and it gets bored? Oh my. If you get caught with your circle open? Oh my.
So, in summary, Summoners can totally work as is, they're just really difficult. At lower levels they have to have prep time. Summoning a beast on the fly is almost impossible. But that's not all a person can do. There are other tactical and magical purposes (opening gates comes to mind as a Summoner side job). But when they GET their prep time, when they go on the offensive, or you catch them ready, they can bring the PAIN. Then it's all about keeping CONTROL of the beastie.
At UPPER levels? Things get FUN. Once you have the shifts available to pull OFF a combat Summon, as in, "Its fine that I'm distracted. It's cool that I don't have time to do it right. I may even have to improvise a few of the ingredients," things get sticky. Your group has to keep you absolutely protected while you summon. You have to stay safe for at LEAST 3 rounds, if not more (more rounds, more prep more shifts?). And here's the best part, you are negotiating at an EXTREME disadvantage, as you are on a very short clock and your possible Hireling KNOWS it. So we have LOTS of potential for serious long-term consequences, and explosive disasters. But if you're bold enough to risk it, and strong enough to try it, there's a lot of power available. And that's what the Dresden Files is all about.