As long as you Bind each receiver, one after the other, you should have no problem with using Redundant receivers. You will still only have 6 channels to work with. I don't think it matters which receivers you use. Most people do this for Redundancy, or because it makes it easier to assemble a complicated aircraft when you can put one receiver in the fuse and one receiver in the wing, for example. Mixing the Throttles on four motors requires the use of Programmable Mixers unless there is a Canned Program in the Tx that handles this.