If it fits...go for it. There is no additional "strain" on the alternator... HOWEVER with smart battery monitors, it may not charge the additional 10ah OR it might detect the battery is on it's way out. If you have access to Peugeot Planet, you might be able to tell the car it has a 70ah battery installed.
I have successfully used an ultracapacitor bank on the 407 in addition to the battery and it didn't seem to mind it at all. :thumb: Ultracapacitors are energy storage devices which can charge and discharge almost instantaneously (over 700-1000amps from a bank the size of your hand), the alternator would see an open ground.. yet it didn't hesitate at all.