Maybe I misunderstood you? I´ll try to answer the best I can. If anything is incorrect in my post, please chime in and enlight us all!
Do you mean that you thinking about getting a 5 pin connector in one end, and 7 pin connector in the other end? How would you solve that?
As you can see in the schematic the pins 4/5 are the ones that are being in use in regard to the midi protocol. Pin 1/3 = NC (not connected), pin 2 = ground (on the outputs).
There are some manufacturers that have been using the non-standard way regarding phantom power using the regular 5-pin connectors. Where pin 1/3 supplies power to their products. However since that isn´t a standard there are many other products around that aren´t compatible with this phantom power method... (it even could mean that this method may ruin your non-compatible products because you can never be sure that these product will have any protection circuit on pin 1/3, since you can´t really be sure that these pins are NC in their products).
Besides that, there are many regular midi cables out there that don´t even have 5 leads within the cable, instead they´re manufactured of cables with 3 leads. Which you may or may not be aware of with any of your midi cables.
Since it´s a bit risky to have a circuit with 5-pin connectors with power supply implemented. Fractal choosed to implement a 7-pin connector instead. A regular midi cable with a 5-pin connector can easily fit into the 7-pin connector, but it won´t ever pass on any power coming from the 7-pin connector in the Axe. So there´s a less risk that your midifoot controller will become damaged because of this phantom power.
There are some midi foot controllers out there that have 7 pin connectors, to be able to be used with phantom power. If you are building your own foot controller, I would recommend that you use a 7-pin connector as well, as you then saves yourself trouble regarding your midi cables. It´s possible to buy a spare 7-pin midi cable in music stores if you suddenly realise that you´re out of such a cable...
/Mike