I've been using a small, lightweight keyboard, fearful that one using a higher voltage would damage my Pi. Could I have invested in a better keyboard or did I do the right thing to prevent damage?
All USB keyboards are 5V. No matter how big or how small, they always use 5V power as supplied by the USB port. However, different devices draw different amounts of current. A typical USB port provides 500mA of current, and the Raspberry Pi (model B) draws between 300mA and 700mA (depending on processor load). Try to keep the current used by USB devices to be less than 500mA to be on the safe side.
Most keyboards are well below this 500mA level, so shouldn't be an issue. If you need to use more powerful devices, consider getting a powered USB hub, so that additional current draw can be performed by an additional (external) power source.