I am in the market for a new gearbox shell.
I know that CA gears dont always work in Sytema boxs with the bearings... id say stick with systema, i dont have much experince with gaurder stuff but systema is my trusted go to brand. Of ocurse a call to AEX will answer your question quite quickly. :D
I dont know if the guarder gears will work with the systema box, they are 'supposed too' but thats no promise. However the 7mm bushings are desirable, 6mm can handle like 35lbs of torque where as 7mm can handle like 60+lbs of torque and allow for speed and torque upgrades beyond what 6mm will handle. Thats assuming everything is shimed and lubed correctly of course. While I believe when things are all correctly functioning other parts will give way before the bearings will (hopefully piston teeth first as thats an easy replacement), it is nice too have the 7mm to make sure thats not the weak part.
What would Brock do?