Yes, the charger voltage must be higher than the battery voltage for any current to flow and charge the battery.
For LiFePO4 the C rating refers to charge/discharge rate and is usually (not always) the same as the capacity rating. A 100 amp hour battery thus (usually) has a 100 amp charge/discharge rate (1C). Most manufacturers recommend a 0.5C rate, it usually gives a few percent more capacity. Most can also exceed 1C for short periods of time (it will cause the cells to heat up). Individual cells usually have a detailed spec sheet that tells you rates, and for some, they recommend a .03C or a 0.2C charge and discharge rate for the maximum capacity.
This is the datasheet for the Eve cells I have:
Most recent Spec Sheet for the 280 LFP cells as provided directly by EVE March-14-2020.
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A LiFePO4 battery really looks like a dead short to a power supply or charger, so a charger will have to limit itself. That is normal for chargers, so don't worry about that. A 6v charger can not be used to charge a 12v battery, individual cells, maybe, but generally speaking, the charger must put out a higher voltage than the battery or no current will flow.
Really, it depends on a lot of factors.