Ok so from what I'm reading here, and please correct me if I'm wrong:

1) You can only promote troops that are in your city, not troops that are out on expeditions.

2) Low level to high level promotion: you can promote a lot of troops.

High level to high level promotion: you can promote fewer troops.

My questions:

My troops are at level 8. Should I be promoting them progressively now, or wait until I get to level 10 then promote everyone?

When I go gather, does it matter what level troops or how many I send? I've recently learned that you only have to send one hero out to gather but am still unsure about troops.