Welcome to the DUC. the simple answer is ANY....and NONE. Seriously, what are you expecting the mixing board to do? It won't work as a controller(a few digital mixers may operate in HUI mode). Mixing ITB(in the box) allows full recall of all mix settings and moves(PT has very good automation). Expanding the number of inputs requires digital connections(you can't just plug in a board with USB or firewire and have it show up in Pro Tools). Using external hardware(in general) with an LE system is sort of a waste of time. Unless your hardware is really top-quality stuff, plugins will give better results(and offer full recall). Analog summing is another story, but that can be done with any of several summing boxes(or a mixer) but I would not be using anything like a Mackie for that purpose(I'd rather use some good "warmth" plugins like some stuff from McDSP or softube). Tell us what you really want, and we can offer more "in tune" advice
BTW, since you have a Mac Pro, I would load that baby up with more internal drives and leave the firewire drives for transfer and backup.