can i use my classic charger for my iphone 4

No, it is not true. There's a lot of nonsense out there, all from people who don't know what they are talking about. The adapter is not a charger, it is a 5 volt power source. The actual charger is in the phone and regulates the charge process, regardless of the output capacity of the adapter. But a iPod Classic charger is identical to the iPhone charger (they are actually the same part number if it's less than 3 years old), so you don't need to worry about it anyway.

Depends how old the charger is. If it is a FireWire charger it won't work, but it won't hurt the iPhone either; it will just give a warning. All USB chargers made by Apple are compatible with the iPhone.

Yes, as long as the cable is not damaged, and on the older cables there is a "Lock" that has two buttons on both sides you have to press to pull the cable out. Make sure you press those buttons before pulling the cable out to prevent damage the the phone.

The charger is not inside the phone, the voltage regulator is. So it doesn't matter if you hook up the little AC adapter or the new 12W adapter that comes with an iPad, it will still charge your phone at the same rate.

The "charger" is the part that plugs into the wall. Most of those made in the last 5 years should charge an iPhone. Older iPod chargers used Firewire and would not work.

The 30-pin cord, which was what the OP asked about initially, should be fine no matter what as long as it's a USB cord.

Yes