You're right to be worried about correcting too much. I don't know how many times I've seen dogs start to show signs of what you're talking about, but were made worse through corrections. Correcting the dog for the behavior is like putting a band aid on a deep wound. You might stop the bleeding for a bit, but then it starts to leak through. You start needing more and bigger bandages, but the bleeding never stops unless you get to underneath stitch it up. Getting to that wound is a heck of a lot easier if you stitch it up in the first place instead of having to go back to clean it up, first.
You have to go back and really evaluate why the behavior is popping up. Dogma had a really great idea of tethering the dog away from you. Changing up the situation will help you figure out what's going on. Is it you? Is it the couch? Is it just whatever space happens to be "his" at the time? You sound like you're already getting an idea of what is going on, so that's good. The more you observe the more you'll be able to figure out a plan of attack.
So, regardless of what specifically Apollo is guarding, you have to think about his mindset of the situation. Guarding comes up when there is something your dog wants, but for some reason feels like there is a threat to his keeping it. If it's a space issue, it's that he wants the area all to himself. If it's you, it's that he doesn't want to share your attention. Either way, the dogs approaching are what is causing the behavior. Something about their approach makes Apollo nervous.
The key is going to be to change his thoughts on the situation. Instead of seeing the other dogs as a threat, you want him to see their approach as a good thing. Since you said he's good about food, this is a GREAT way to work on things. Get something really, awesomely delicious and keep it on you when you're in a situation that may cause the behavior. When you know the dogs are going to start acting in a way that makes it occur, cut it off at the pass and start shoveling treats in Apollo's mouth. The idea is that you start to rewire his brain to thinking of the dogs coming around as a good thing. Ideally, instead of turning to fend them off, he starts to see their approach and look towards you for something good.
In addition to this, you want to stay really on top of the dogs' dynamics when they're around each other or playing. If you start to see situations where a problem might arise, use yourself and the dogs' obedience to defuse the situation. Something as simple as walking between the dogs or calling someone over to another area in the room can completely change the tension. It's a little like you would do with dogs and kids. If someone seems too rambunctious, or someone seems like they're getting nervous, calmly break it up. Don't forget to reward good behavior here, too! Like you were saying, having the dogs know "place" commands is really great for this. It's also a good idea to have different areas in the house gated off, if possible, so you can break up dynamics if need be. That way everyone has a chance to calm down. That's the key to everything, really, calming down the situation. By throwing in corrections, you're taking a dog that is already on edge and making that nervousness go up. They might stop the behavior, but nothing has happened to make them feel at ease in the situation. You really want to get into a place where if a dog feels uneasy, the first thing they do is look to you for guidance/help rather than feel the need to figure it out for themselves. You know that look when a dog is worried about someone or something and looks at you with eyes that are saying, "You see this? What are you going to do about this?" You want to be able to see that look and respond accordingly.
This whole plan of attack takes a lot of finesse and understanding of dog body language. You have to be observant and vigilant. If you have a hard time seeing the red flags, or if it's just too much to pay attention to at once, you might want to consider breaking up the dogs into smaller groups via baby gates, or possibly having a trainer (who isn't going to have you giving corrections) come to your home to see things first hand. Sometimes having another set of eyes really helps you notice things you're missing, and that's all it takes for you to get a handle on things.
Good luck with everything, keep us updated!