you can't just have everyone paying for goods with whatever they wanted
True: You can't just pay with whatever you want. For example, I doubt anyone will give me car in exchange for a tuft of my hair. What is offered must be of sufficient value. However,...
I agree it doesn't have to be gold, but it has to be 1 or 2 selected items that everyone agrees on.
False, kind of: It need not be so restricted. See here. Everyone could own any number of commodities and other assets. Buyers could be willing to pay in a number of assets, at a given exchange rate (maybe just market rate with a price limit set to protect against temporary market insanity). Sellers could be willing to accept payment in a number of assets, again at a given exchange rate. The negotiation could be handled electronically -- it just needs to find one or more assets buyer and seller have in common. (Ideally the buyer and seller can express preferences about what they want to unload/acquire, so the benefit of the transaction to both parties can be maximized.)
Of course, it's possible there is no commonality, in which case an intermediary could be used, but that might add some expense to the transaction as in that case it's not just a straight-forward change of ownership. E.g., the buyer may only have silver and soybean futures, but the buyer only accepts wheat futures, platinum and gold. In that case you then go to the buyer's and seller's list of "acceptable intermediaries". Maybe platinum is on the buyer's acceptable-intermediaries list, so some of the buyer's silver is exchanged on the open market for platinum, and then the platinum ownership is transferred to the seller. In theory you could automatically/instantly set-up and execute whole chains of such exchanges if the buyer and seller are only willing to deal in (different) very limited and specific assets that very few others on the open market deal in.
With cheap automation and a widely-available world-wide network, the old "rule" of needing just one currency everyone uses doesn't necessarily apply anymore.
If a company spends 1 American dollar whether its back by gold or not to make a candy bar in a factory [...] you have taken a dollar out of the economy and you can't put it back
Sorry, but you have just experienced a "word problem math failure". You say the company spent 1 American dollar. That means they took the 1 dollar they had and gave it to someone else (in exchange for something). No dollar has been taken out of the economy. If a supermarket wishes to accept fertilizer as payment, that is perfectly fine. (Hopefully that's all done electronically...)
Gold is very hard to melt
Actually, platinum is very hard to melt -- which is why ancient peoples selected gold instead of platinum. (But "hard" is relative.)