Can you get matched against a player on your blacklist?
Currently, yes. Quickmatch ignores blacklist. Though I could see this feature possibly coming some day.
there will be no pre requisites ?
Correct. Quickmatch uses its own matching based on rating, it does not look at prereqs.
What's the formulas behind the ratings? I seem to get +10 or -10 for a win or loss, but I've seen people having ratings involving decimals.
It's trueskill, but the most you can gain or lose is +10 or -10. Right now there are few players so you're often getting matched with people with far away ratings, but once there are lots of people you should usually get somebody with a more similar rating and rating changes will be smaller.
How does the matchup system work?
It matches you based on a "match rating" which is computed, mostly based on your rating for that template. The idea is that this will be people's first exposure to new templates, so even if they're good on one template they shouldn't be matched against good players on a new template.
Also, it'll never start looking for a new match while it's your turn in a quickmatch game. This is why, even if you set the number of games at once to 5, you'll only get one at a time. Unlike the ladders which just dump 5 on you at once.
Also, the longer you've been waiting for a game, the greater range in rating the quickmatch system will allow. When you first start looking for a game it will match with someone with a very similar match rating, then the range grows over time.
is the chance for a FFA really low?
If multiple players waiting for a game with a 4-player template checked, the matching system will prefer the many-player template over a 2-player template. However, as you suspect, it may be rare to have this situation if they also have 2-player templates checked as those will start before 4 players accumulate.
The best way to start a 4-player template is for at least 3 people to check only that template. The 4th can have 2-player templates checked.
Even without doing that, it can start if the 1v1 match rating for a few people is far apart, so 1v1s don't start, but the ffa rating is closer together.
And can you put in a higher number than 5 manually to get more matchups?
You can put in a number up to 10. 10 is the real maximum.
How will the template voting work?
We can start talking about voting soon.