1. Sometimes I get an option to add a new character, I've added all of them so far, and now I have 12 total. Should I deny all new character or add all of them? If I add some, which ones should I add?
In FFTA, new characters were the average level of your team, but now they're capped at 30 so having a lot of characters doesn't really cause any problem. IT's good to have a diverse clan with everyone learning abilities, if only so you have jobs available for dispatch missions. (The Wanted: Whatever dispatch missions require specific jobs. It's up to you to figure out which ones. :P) Aside from Luso and Adelle, there are like 6 special characters, so it's good to have slots open for them.

2. All my character have only 1-2 abilities. In order to get new abilities I have to get a piece of equipment that they can equip and that has an ability, then level up the ability until I can use it. The stores don't sell regularly much equipment that I can use. Apparently all the good equipment exists in the Bazaar. But there are like a hundred different items! How do I know what item has what power level and what ability and goes with which class?! Maybe I missed something, but I don't understand all of this yet. What's your thought on this?
You could use a guide if you wanted, but it's not really necessary. :P

What I did was I only made new items that I was probably going to use in my main party. Like if I didn't have a Seer at the time, I didn't make any books. The higher ranked items are more powerful than the lower ranked ones, obviously. The game lets you preview an item before you make it (what it looks like) so if you see something you don't want, you can cancel out of it. Don't worry about it too much. There's plenty of loot out there. You can use common sense to get a general idea of where to find certain pieces of equipment. Obviously, the rod that teaches Blizzaga is going to have a description of an ice elemental piece of equipment.

3. I have only done the first level of the few Clan Trials I seen, do I really need to do them? I don't see a use for them yet. I believe that eventually I'm going to come up to new quests that require me to have certain skills and in order to get those skills I have to do Clan Trials, and that's the purpose of doing them, right?
The clan trials are completely optional. I maxed out all of my Clan stats without even having a title. :P However, for every level 1, level 3, and level 5 rank in each trial, you get a Clan Privilege. Some of these are absurdly useful. For example, if you do the Levels 1, 3, and 5 matches for Aptitude, Adaptability (it's one trial named that, not one for aptitude and one for adaptability) you get Channel MP, which doubles the amount of MP you get on ever turn. Combinging that with Half MP, you can actually cast Illusion spells on every single turn. My Sage can cast Ultima every single turn, and my Gladiator and Hunter can cast it every two turns.

Some of the more advanced titles give you extra little bonuses, like discounts at shops and stuff.

Also, in Final Fantasy XII Montblanc mentions that he had a teacher/master who died or something while trying to defeat Yiazmat right? I believe he said that he had many siblings. I think that these siblings, Montblanc, Nono, Hurdy, etc, represent some of the main team members of the Yasumi Matsuno team.
I don't know anything about that. :P