1 & 2) It depends on the items and quality.

For general purpose stuff, such as raw lumber/ores and whatnot that are only really useful for crafters to turn into items, big ol' stacks of 99.

However, for things such as items that are relevant for say, Company turn ins for leveling gatherers, putting up HQ things in stacks that are relevant for that works well. So that's 10 Lumber/Ore and 3 Fish (Unless it's a Mooched fish then it's single item)

3) Either sell in the one with the cheapest tax rate, or sell in the one that is most populated on your server (Which means no taxes for people buying from there). For example, Limsa Lominsa and Kugane are quite popular on my server.

4) Depends on what you're farming and if you're getting a lot of HQ or not. Not to mention, your servers economy.

5) Personally, I tend to put like items on the same retainer. Just for ease of my own reference. So I don't accidentally start like undercutting myself >.>