This is a question for anyone that has played or run a game using Drop Dead  Studio’s “Spheres system”. I’m working on a homebrew setting using the system and have a concern. It may not be a legitimate  concern, but you can tell me that, since its part of what I want to know.

My concern is this, does champion of the spheres allow for characters that greatly overshadow characters only created with spheres of power, or spheres of might? If yes, how did you, or how do you suggest addressing the imbalance?

First: When in doubt, players should use the Champion version of any Power class that has one. There's no point in a Mageknight who isn't a Champion. If you're a magical knight, of course you're going to want martial as well as magical abilities. Likewise with Elementalists.

Second: The Sage class can be pretty durn powerful if optimized properly.

Other than that, in general, no. Without a special ability or a Technique (and the latter is generally a major SP or action tax) you can't combine magical and martial spheres together. A purely non-martial spherecaster is probably a full-caster. A purely martial character has a broad spectrum of nonmagical abilities and can often deliver caster-like levels of damage from pure combat. Either way, you're still limited in the number of spheres and abilities you can learn on any one character.

I'm not really familiar with Guile yet.