It works out to something like 1 extra follower per four points of Charisma. Also, having "expert" training in the Persuade skill (which requires having 3 levels of Persuade, and finding an expert to train you - Madame Lil in Tarant can do this) you get an additional +1 follower. You don't want to spread yourself too thin, but by using a few tricks I was able to master four tech disciplines by game-end, and dabble in three others, while still having decent stats. Here are a few helpful things to know:
-Each level you buy in a tech discipline does two things. One, it gives you the schematic for that level, allowing you to craft that item. Two, it increases your "expertise" in that discipline by about 10-15 points, which is necessary to craft more advanced items. However, you can also boost your expertise by buying the appropriate manuals from Tarant university, and multiple manuals stack the bonus. This means that the only reason why you would ever need to put points in a tech discipline is if you want the schematics that come with them - buying ranks just to boost your expertise is a waste of points.
-In order to buy ranks in tech disciplines, you need a certain intelligence level. However, potions of intelligence will temporarily give you a massive boost to this stat, I think it's by 10 points. Since you need 19 Int to unlock the highest levels of the tech disciplines, you could in theory have as little as 9 int and still unlock any tech level you want - just down a potion, spend your character points, and when the potion wears off you will still have the schematics. Your "expertise" will drop to the level your normal Int can support, but you can boost it back up with manuals, or by drinking another potion of intelligence. Eyegear will give you +2 PE while equipped, though you can find these in-game. Goggled Helmets give +3 PE. Charged rings give +2 dex each (which is amazing for how cheap they are.)

There is a one-time-only craftable item that will boost every stat by +1, but you won't find the schematic until almost the end of the game.

Another source of stat boosts is the Arcanum Gods Quest. You've probably run into some altars to various Gods scattered around Arcanum? If you present the appropriate offering, you will gain that God's blessing, usually a boost to one or several stats or skills. The trick is finding out which order to do it in so that the other Gods don't curse you. There is a necklace you can steal off an Elf in Stillwater that gives +2 Beauty and +1 Cha. There are also two quests that give you a permanent +1 Cha bonus (Madame Toussade's crystal ball in Tarant, and restoring the Statue of Geshtianna in Stillwater). Plus, some of the Gods in the aforementioned Gods Quest will give you Cha buffs. Firearms hit rate is dependent on a number of other factors too, including your Perception stat, your distance to the target, the light level, whether there are any obstacles between you, etc. Expert training in Firearms does not improve your hit rate, it just reduces the penalty for making called shots. You can only use shovels on certain graves - you will know because they will have a mound of dirt over them, will light up when you hover the mouse, and if you click on them it will say something like "you try to dig in the dirt with your hands, but you don't get anywhere". I think there are only 2 or 3 diggable graves in the game, all near the end of the game, so feel free to drop that shovel in a crate somewhere and pick it up later. Manuals improve your expertise, allowing you to craft more advanced items. However, they do not improve the quality of the items crafted - you can either make them or you can't. There are three craftable items that can be sold for a pretty substantial profit: flash grenades, explosive grenades, and eyegear. The components can be readily bought in shops, and if no-one else will take them, the junk dealer at the docks will still buy them off you for a hefty sum. Only way I've found is to take the other items away from them. Annoying, I know.

Note that followers will never equip a weapon that requires ammunition until you give them some of the appropriate ammunition. Took me a while to figure that one out. I don't think so. If your characters start wandering off, it's usually because they've seen something sitting on the ground that they want to pick up. They'll return to you once they've grabbed it. Yes. Bashing open doors or chests with weapons damages those weapons. Axes are mostly resistant to this, though, so if you have an axe, use it to do your bashing. (And by the way, if you're a technologist, keep checking the stores for the "Pyrotechnic Axe" schematic. It's bad-ass.)