World-of-warcraft – catch Lunkers in the Garrison

What used to be the minimum required skill to fish in any given area is now the minimum required skill to not catch any junk. Once you meet or exceed this minimum for a zone, whether natively, or through the use of equipment, buffs and consumables, you will no longer catch ANY junk. If you are below this skill level, the chance of catching junk is proportional to just how far below it you are. The further below the threshold, the more junk you'll catch.

The exact formula for the chance of catching a fish when your skill is below the threshold is ((current skill/threshold skill)^2).

For example, the Dalaran Fountain has a no-junk skill of 525. As we can see in @GreatBigFish's answer, this junk threshold is above the (unbuffed) WOTLK maximum skill of 450. Fishing at max skill in Dalaran with no lure would yield a 'fish' roughly 73% of the time. The other 27% of the time, you will catch one of the 13 'junk' items found around the world, all of which vendor for mere copper. To never catch junk in Dalaran, you'd need to use a +75 lure or better at 450 skill.

There is one notable exception to this rule: You will never catch junk in a pool. Fishing from pools gives a 100% catch rate on whatever type of fish spawns in that pool. This has caused many to claim that actively leveling fishing skill is now pointless because it provides little practical benefit - fishing from pools is always more efficient anyway.

For more information, El's Anglin' provides a tremendous resource on the subject of fishing.

There actually is a list of all followers in-game. You can find it if you click the Garrison Icon next to your minimap and then select the "Followers" tab on the bottom of the window. The ones you haven't unlocked yet are greyed out but there is a description of how to get them (Achievement, Quest etc.).

The in-game list only shows the ones you can actually unlock since some are faction-locked. For a complete list I'd suggest looking at the Wowhead Follower Page.

While you can unlock every Follower that's listed in-game, you can only have 25 active at once. This requires you to have the barracks upgraded to level 3, otherwise it's only 20 active at a time.

Regarding your question about the optimal followers to send on a mission:
There are multiple things that influence the success chance of a mission. Your follower's special ability is by far the one with the greatest influence, but things like the level of your follower and the environment are also important and can lead to a higher success chance if countered correctly. If a follower has some special ability that's useful on a quest an Icon with said ability will show up next to him if you send him on a mission.