Welcome, Pete. What kind of music are you interested in making with this?
First things first, you do not have enough voices to make use of Harmonaig. Marbles already includes a quantizer, and Plaits has a pseudo-polyphonic chord mode, so Harmonaig will be a fairly expensive paperweight in a rack of this size. The Qu-Bit Chord v2 may be a suitable replacement as it has similar functionality and includes built-in oscillators (it's also smaller).
I've been using ALM's mmMidi module for MIDI input. That may be an option to check out.
Otherwise, I would just encourage you to consider if the Mutable Instruments user interface is right for you. Their elegance and sound quality are great, for sure, but I personally don't get along well with some of the opaque button combos and different colors of LED mean different modes and functions, etc. Others here love that way of working, or at least tolerate it.
Have fun and good luck!