Hi Timmo, both pikemen and musketeers could alter their order between both rank and file depending on circumstances. For example, while at March, order between ranks would extend to six feet, and should pikemen be instructed to adopt the cumbersome 'Charge for horse' posture they would be expected to move to their closest order between files. However, for all practical purposes and for the majority of the time three feet between rank and file was the norm.
For musketeers giving fire by rank or by file was used as default. Yes, you could double for the purposes of giving a salvee , but this was a singular event, and once delivered the unit was left with very limited options – flee, fight or reload. The first two had very unpredictable outcomes but the third would be done by reforming the body as it was before, ie in its original ranks in its original order.
Pikemen in combat are harder to pin down as what they actually did is not clear cut. There seems to be two schools of thought. One is the fencing theory where two bodies stood off each other and jabbed vigorously. In this case, retaining order between files makes sense as fencing with a pike requires lateral movement by the fencer and closer order would hamper that. The other option of a reenactor style spiky rugby scrum would require a very close order, but I'd suggest strongly that this would have only happened just prior to contact, and that should the units back off an reform, again this would have been at order.
With regards to gaps in bodies, you have to think of an ECW battalia not as a single block but as a number of companies/squadrons/bodies working in concert. For example a wing of 150 musketeers could work as three distinct units. There would be a six foot interval between each of them. This would then leave a lane for each unit's musketeers to walk down having given fire.
Does that make sense?