[Stratos]

Yes, Champion of Torm is good. I believe it's the class that can do the most damage with one hit, something like 2000 points of damage, using Smite Evil.

As Micah said, no multiclassing with any class will lock you out of your other classes, that's and old 2nd ed. rule. You can take any class up to a maximum of 3 classes, as long as you meet the requirements of those classes.

The thing you mentioned about the classes staying within one level of each others has to do with experience points penalty. If you have a multiclassed character and none of the classes is the favourite class of your race, then you'll recieve a 20% XP penalty if the levels of the classes is more than one level apart. Humans and half-elves has 'Favoured Class: Any' which means that the class with the highest level is your favourite class if you're multiclassed. This means that you wont recieve a XP penalty while playing those races as long as you only have two core classes.

Prestige classes never gives you a XP penalty, so you don't need to worry about that.