I've never heard of a 6v receiver, but I don't claim to know all of them. Which RXes require 6v? I'm just curious.

Any RX (that handles the voltage) will work with a BEC. In the same way, you can plug a normal battery into any channel opening, (most) RXs just don't care where the voltage comes from. I often plug the battery into a random channel opening instead of the "Battery" plug. Some RXs don't even have a dedicated battery plug, they just expect you to use an unused channel, or a y-harness.

The biggest issue with using a BEC is the "2A" part, and the way the BEC drops the voltage from the flight pack.

First, most BECs are only rated for 3 or 4 servos. That's the "2A". If you use digital servos, or put a large load on several servos at once, you can draw a lot of current. Draw too much, and the BEC can't deliver it all, and either something burns up, or the voltage drops to the RX and you loose radio contact for a split second (or longer, depending on the RX and BEC).

Second, many ESCs have BECs that can be used if you're using a 2 or 3 cell Lipo pack, but not if you're using a higher number of cells or they have some other limitation. The reason behind this is that the BEC built into the ESC isn't always the best at dropping down the voltage without wasting a lot of power and creating a lot of heat.

There are seperate "switching BEC" devices out there that will handle larger servo loads and higher input voltage. They are using better design and componets to handle more power though the system without burning up.