I have a character Rogue1/Wizard1 (favored class rogue). Since a rogue has light armor proficiency, is this also valid for wizard spells that I cast with somatic components? By "valid" i mean: Can I cast a spell with somatic components wearing light armor without a 15% risk of failing?
Nonproficiency in armor means that you take the armor’s armor check penalty as a penalty to all sorts of things it wouldn’t normally affect, including attack rolls.
By virtue of being a rogue, though, you have proficiency. Thus, you do not take these extra penalties, and the armor check penalty of the armor only applies to the usual skills.
Note that an armor check penalty of 0, as is the case for most light armors (but not the best one), means that there is no penalty for being nonproficient in its use.
But none of these things is particularly relevant to the wizard. The wizard could take a (at worst) −1 to a bunch of stuff, and it could conceivably be worth it. After all, many of his spells won’t care. What they will care about, though, is the arcane spell failure chance. That gives him a fixed chance to fail any spell, no matter what, and being proficient or not makes no difference in that.
So even though your rogue/wizard is proficient in light armor, he still shouldn’t use any. The arcane spell failure just isn’t worth it, nor is setting a feat on fire with Arcane Armor Training. Mage armor is a fine spell, and it lasts for hours; one (or, at low level, two) casting should see you through the day.
If you have light armor proficiency, you are free to wear armor as a wizard. The big risk, however, is Arcane Spell Failure
So, for example, if you were wearing leather armor, each time you cast a spell, you would roll a d10. On a 1, your spell fails.
You can reduce this by 10% as a swift action with Arcane Armor Training, which for most light armors mean you would suffer no penalty.
Multi-classing in 3.5 and Pathfinder have some drawbacks, including Arcane Spell Failure, a lower caster level than your character level, lower spell levels available, et al. Some base classes from expansion books get around these limits, like Beguiler (3.5 PH2) or the somewhat awkward Spellthief (3.5 Complete Adventurer), but they trade them for other traits like the Beguiler's extremely limited spell list and loss of sneak attack (although it's sort of traded for cloaked casting), or the non-existing spell list for a Spellthief. I'm not aware of a Pathfinder-specific Rogue/Wizard base class from an expansion book, but if your DM allows 3.5 expansion-book-base-classes, it might be worth looking into.