Yes... that's will work. This is a 5V coil active low. You can use a 5V coil active high or 12V active low too. Any active low boards get the switch set in the Device element's settings. 

As you said, you can always remove it when you go 50A.

This is what I do... only I use the 16 relay board and use 2 relays for the element power .  I use the NC contact one one element relay to interrupt the power of the other if its on.

Is there a benefit to using 2 versus 1 relay?

I am not exactly certain what @Augiedoggy is doing here, so I cannot comment. Perhaps he can post up a sketch.

In the schematics we put online, we use one relay to switch power between the element contactors. These contactors are dual pole and switch both L1 and L2, so there is no extra pole set to use.

Which I believe is similar to what I was proposing but rather than 5v powered form the board 12v from a power supply.  Correct?

Sorry... not following you. The "element switching relay" is there to ensure only one element contactor is possible on at a time. The SSRs are ultimately controlled by the MEGA and BruControl, but it is very possible to turn them both on simultaneously, so this prevents that. It is switched by BruControl. You could have it switched via a manual switch, but that defeats the purpose of the automation / digital control panel.

In our schematic we have its contacts switching the high voltage signal from the E-stop. The contactors (1 for each element) it feeds require 120V coils in that circumstance. The relay can be in one of two positions, so only one contactor is engaged at a time.

If you want to use contactors with 12V DC coils, that is fine, but then you cannot feed its COMMON terminal with a 120V line from the E-stop. In that case, you would need to determine what the E-Stop will actually switch.

If you have a schematic, please feel free to send it to us via email.