I have been doing some testing and have found an alternate way to get Solaire. Its just more annoying then using the Chaos Servant because of all the back tracking involved. I am far from finished in testing what can lead to not getting him but I have tested this method twice and found it words so far.
This method works by not advancing Solaire's quest line past a certain point until its safe to do so. I Stop it in Anor Londo by not talking to him or summoning him at all(next I will test whether one can summon him in Anor Londo and not have his quest advance), which also leads to him not being available for summon at the Centipede Demon. I simply ignore him until I have visited Lost Izalith and killed the Red Eyed Chaos Bug and opened the doors. Once I have done that I go back and talk to him in Anor Londo. This is where it gets annoying. Because you have to talk to him at the following locations in order to get his quest to progress. Sunlight Alter. The bonfire after the area you fight the Centipede Demon in. Then where you kill the chaos bugs. You have to go up the stairs first to get him to spawn if you choose to come the quick way through the door. I don't know why. He just never spawns for me until I have gone up the stairs and back down them. Anyway if you follow that chain of events you should get the conversation with Solaire that lets you summon him for the boss battle with Gwyn.
So, yeah, that is alot to do isn't it? But it works. So its up to you whether its worth the pain of so much backtracking just to summon Solaire or not. I posted this because I always wondered if it was possible to get Solaire for the final battle without the Chaos Servants and once I figured out that it was possible I thought that other people who would like to use Solaire would like to have this option available to them. Hope it helps people out and adds a little bit of more enjoyment for my fellow DS players.
Feel free to comment, especially if you guys figure out more of the conditions that go into this.