[RPG] Does taking levels in a prestige class cause XP Penalty

Sure you can, as long as you meet the prestige class’s prerequisites. The Commoner class does not have any special rules about its advancement, it’s just a particularly weak base class.

It’s actually the easiest way to enter the Survivor prestige class (Savage Species), which requires that one’s highest base save bonus be lower than one’s character level. In a similar case, the Adept is the easiest way to enter the Hexer prestige class (Masters of the Wild), since it requires lightning bolt as a divine spell, and neither clerics nor druids get it, nor is it found on any Domain (amazingly).

I would comment that a common way to handle games where you start as Commoners is to have you “trade in” Commoner levels for PC-class levels, to avoid you taking pretty serious hits to your abilities relative to your nominal ECL. This is a non-issue if your DM knows how to compensate, but it might be worth mentioning as an idea to your DM.

Your Familiar only improves with Sorcerer levels, so a Sorcerer 10 has a stronger Familiar than a Sorcerer 5/Prestige Class 5 (unless, of course, the Prestige Class explicitly advances your Familiar, which a few do – Alienist from Complete Arcane and Fleshwarper from Lords of Madness, for examples).

But other than that, no. Sorcerers generally have nothing to lose and everything to gain from going into a full-casting Prestige Class.

Of course, be careful about prerequisites; a number of them can be quite painful to Sorcerers (who lack bonus feats and cannot trivially pick up the ability to cast a given spell the way a Wizard can). A classic example of this is the Loremaster: in theory, the class is 10 levels of free class features for the Sorcerer, but in reality you have to burn several feats, somehow get 8 ranks in cross-class skills, and find seven different Divination spells worth spending your extremely-few Spells Known on. (The Wizard, on the other hand, can get in while barely trying, thanks to his class skills, Bonus Feats and arbitrary number of Spells Known.)