You will either over-extract or under-extract the coffee.
Over-extraction occurs when you run too much water through and you will be able to see this as the coffee turns pale and eventually you will end up with almost clear (or barely tinted) water running through. Over-extracted coffee tastes bitter. Really badly over-extracted coffee can sometimes exhibit metallic or plastic taste.
Under-extraction is a much better option. The resulting coffee will be thicker, stickier and less bitter. The downside is that you are not getting the maximum utilisation from the beans and you could have made more acceptable coffee with those beans.
A ristretto is a short extraction with minimal water and ideally at a slightly lower pressure. An espresso should be the same coffee, but the extraction would use more water and often a slightly faster flow rate / higher pressure.
The caffeine content will mostly be dependent on the beans used and the quantity of coffee in the pod. A longer extraction may increase the caffeine content slightly, but not enough to be noticeable.