Olav, Tom did not mean a tiny army in the sense of "single man army". He just meant a small army, able to take a single day's fire (or shock at low tech). This is, at least in theory, computable beforehand.
The reason to keep it as small as possible is that a force in combat gets the weighted average morale of all armies that enter the fight. So, the smaller the proportion of the small army to the main army, the higher your morale will be.
One thing you can sometimes do in this end, particularly at lowish land tech (9-13, or down to land 7 with clever use of artillery), is send the main army to arrive on day 5 of the battle. At low land tech, firepower is very low, so few casualties will be caused. But morale damage is still quite pronounced. So, you get the whole fire phase to knock down the rebel morale, then any morale losses you've taken are wiped away with the arrival of the main force.
According to the wiki the chance will increase with 2% if reducing population to below certain levels (5000,10000,15000 etc).
Huh? Oh, I see... yes the wiki is a little poorly worded. No, it's not quantized in 2% chunks; the chances increase smoothly with pop. The 2%/5000pop is really 0.0004%/pop. As to whether or not the final chance is rounded or not (that is, is a displayed 33% chance really exactly 33% or is it sometimes 33.4521%?), I don't know. It would be a hard thing to test.
I'm not sure what you mean about protecting manufactories other than the obvious, that mercs can be recruited fast if nothing else can get there and a rebel army is marching in.
But in any case, something like that should not go in the mercs article; it's too specific. I'm not sure there's any obvious place to put a little tip like that.
There is no specific thread here to discuss the wiki. At least, not currently -- there have been a few in times past. In my opinion, everything discussed here relates to the wiki, in that the wiki should be accurate and complete at least in terms of describing rules. (It isn't quite there yet, but it is the best thing out there.) Ideally it should also have strategy articles discussing "big picture" strategy and useful tactics, and "opinion" articles like the country guides, where reasonable people will differ.
I've put up a fairly brief set of guidelines for wiki writers, at the writer's guide article, if you're interested.
http://www.paradoxian.org/eu2wiki/index.php/Writers_Guide
If you have some specific thing in the wiki you want to talk about, make a new thread. Or for a more general discussion -- we've got plenty of threads here.
For alliance leadership devolution, I think the order is the order they join the alliance, but not sure.