1-The bonus is on a planetary basis, regardless of whether or not you're Torians/Super Breeders. If a planet is at 75% approval, it grows at a 1.25x rate. At 100%, 2x. At 100% with Super Breeder, 8x. It's really that simple. Your overall approval only matters for elections (and, arguably, only two weeks per year, as they're biannual).

2-That depends on what your morale bonuses are, both racial and structurally. The higher the population, the more of a penalty you incur for the morale for that planet, as well as a penalty to the increase that racial morale and morale structures bring you, up until 25B, where it caps at a -90 penalty and a factor of x0.1. (Note: This is -80 and an x0.2 factor for DL.) For instance, I have one Virtual Reality Center on a planet with 25B people. The VRC gives me +40 morale, but since the population introduces a 0.1x factor into it, I'm only getting 4 from that VRC. Without factoring in racial bonuses or taxes, I'd be at 100-90+4 for a total of 14 approval for that planet.

The Wiki is very good, as Zyxpsilon has suggested, albeit a little outdated-though most of it what it has does still apply to DA, even though it was primarily written for DL.

3-In larger maps, trade can be a significant source of income, particularly when you add in economic starbase clusters with trade enhancing modules. If you're just looking to increase your overall income, for instance for score, that may be worthwhile. But it's most definitely not worth it to have trade routes in favor of planets, in almost any game. Influence can be a large factor, particularly as an economic strategy will almost always (I can't think of one that doesn't) include stock markets, which also grant a 5% planetary influence bonus. It's probably better to go for influence than trade, though. One thing to keep in mind, however, is that trade routes do strengthen relations between races, and this helps keep you out of more wars than is necessary.

I feel like I haven't really answered your questions, except for the first one, but that's the best I can do.

One farm per planet. None on civ capitals (your homeworld, homeworlds you take, minor races' worlds). Two farms on colonies after you get all your morale tech/trade goods in gear, and snag all the morale resources you can. Again, none on civ capitals. Particularly in smaller games, you won't get enough morale resources to justify putting two farms on a planet.

Note: When I say "farm" I mean Xeno/Intensive/Advanced. I don't count the basic farm; Xeno is cheap enough (tech AND building cost, in fact) that you should never use the basic one.

But yeah, feel free to fill up the rest with stock markets. Don't forget to keep a production planet or three, though...and the same goes for research.

It seems you're now asking about TA specifically. I have yet to try every race in TA; my advice is just general, and in fact probably more specific to DA than anything else. Maybe I'm misinterpreting that, but due to the fact I haven't played TA Torians I have no idea what kind of approval structures they have, nor what you're getting at when you comment on it.

But I'd say if you need more than one morale building per planet (DA example: VRC), you need A) more morale resources or less people on that planet. Possibly C) a bigger map might help as well.

Also, don't be afraid to tax your people. You only need planetary approval to be at a minimum of 41% for normal growth to take place, and you only need about 60% overall approval the single week of an election twice a year. You can even go as low as 21% planetary approval (not on all of them, as some will be higher, as in the above example) and have your population neither grow nor drop, but I personally don't like doing that.

Of course, if you're using the Torians for their Super Breeder ability, you'll want to ensure you can make use of it, though generally dropping taxes will work.