Can't help too much on the magnets - I tend not to think about them - but there's others who can help (and I think there are old threads with loads of info - use the search facility, you'll find loads of stuff). I think that the lower the value, the "softer/warmer" the pickup sound is - but don't quote me...
A plate on the bridge pickup fattens it up. It's more common on a tele bridge, but I understand it can do a similar job on strat pickups. I guess the tone must change a bit, but I think it's a fattening rather than losing anything. I decided not to bother when I got my Irish Tours - and as it turns out I really didn't need it, they are nicely balanced, and I'm using a strat bridge on it's own for the first time in my life.
I personally went for ITs because I was pitching for SRV and Rory Gallagher territory. However, I'm fairly sure that others are getting SRV tones out of Apaches on here - and I've heard elsewhere an argument that SRV didn't actually use hotter wound pickups anyway!
For a "fruitier" Little Wing sort of tone I'd be aiming at something other than my ITs, although they're not bad at it. I believe that Apaches or Mother's Milks would be good choices for that. From what I've read on here, the difference between them is roughly "Apaches for 50s spank" and "Mother's Milks for 60s quack".
But like most people, I've found that BKPs are very versatile - for example, my ITs do most of what I expect a strat to be able to do. I picked them because I wanted to use them mainly for slightly hotter blues stuff, but still be able to cover other bases. They seemed the obvious choice, and they've not let me down.
If I was after more of an allrounder now, I'd pick Apaches or MMs.
EDIT: Philking posted while I was typing (I'm also cooking the sunday roast, so it's going a bit slow :D) - I think it was him that gave me the 50s/60s spank/quack ideas
