Why were both gold and silver used as currencies and not just gold.
Realistically there was not enough gold to go around
No that had nothing to do with it. Silver (and Copper) were used because there is a limit on how small gold coins can be used in any practical manner.
The US gold dollar for example is only 75% of the diameter of a dime and weighs about half as much. Imagine how easy it would be to lose such a coin. The smaller a coin is the easier it becomes to counterfeit or reduce. Scales (especially in 1850) are only so accurate. A coin 0.1 grams light would be very hard to detect but with the US dollar weighing only 1.6 grams it would mean almost 4% of the value had been shaved off. Worse the average wage in 1850 was $0.18 so a gold dollar represented about 5.5 days of labor. That would be like today having a coin smaller than a dime worth ~$700. Now try to imagine even smaller gold coins and imagine doing so with technology (for purposes of minting, validating, exchanging, transporting) circa 1800s. How would you pay for a penny worth of nails with gold? Someone is going to mint a 0.1mm gold coin (that is the diameter of a human hair by the way).
Coins with more base metal were developed because given the constraints of physical tokens it was easier to use larger tokens of less value.
If you total the number of dollars in circulation I am sure there are more cents in circulation than satoshi's even when all 21 million coins are mined. If you then totalled all the world currencies and the smallest division of that currency it would dwarf the number of Satoshi's. Basically there are not enough bitcoins to support a world economy. So unless the upper limit is changed a complimentary currency would be required.
The amount of global currency in on the order of $4T USD (
http://en.wikipedia.org/wiki/Circulation_(currency) ). That would be ~400 trillion pennies ("cents"). Bitcoin has 2,100 trillion Satoshis (21M BTC * 1E8). If Bitcoin hypothetically replaced all global currencies (a pipe dream but we can consider this an upper limit) that would put the value of 1 BTC at ~$200,000 USD (2012 dollars) and the value of 1 Satoshi at 0.2 US cents (or 5 S = $0.01).
Still, the divisibility of Bitcoin can be increased if necessary with a hard fork. If there ever came a time that such increased divisibility was necessary it would be a fork that could be made with overwhelming support (it preserves the value of existing holdings in a non-inflationary manner). It is unlikely more than 2 quadrillion units will ever be necessary though. Even today there is little need for divisibilty of the dollar to the 2 decimal places. It rarely affect pricing. Have you ever decided not to buy something because it was 1 cent more than you were willing to pay. The cent simply lives on due to inertia. Many other countries already use currencies with less divisibility without any adverse effect.
BTW I am not saying Bitcoin will be the only cryptocurrency, it may have long term competitors with significant share or it may be replaced completely by something superior, or we may see specialized cryptocurrencies develop which carve out a niche (i.e. a CC designed around micro transactions, a CC designed around improved anonymity, etc). Regardless of what the future holds it won't be because Bitcoin needs a "silver" or that there aren't enough coins to go around.
