Are the module spots pre-determined? Can I install the modules in any spot they'll fit, either fore or aft?

Module spots are not pre-determined. Modules can fit anywhere on the 3x2 grid not blocked by another module. For an example of this: the Olympic mounts two sets of biodome pairs; the domes are pretty large and the reason you can't mount three is that the space that a third would need is crowded out by the other two. You could mount a different 2x1 between the Bio-Dome modules as long as it's one that has a small footprint.