I'd also be interested in more info on how the 'trade network' bonuses work.
This is from memory as I don't have the figures on hand right now, but I did some testing the other day and came up with the following:
* The longest trade route is not actually the longest possible trade route you could imagine - it's complicated

.
* The numbers you get for credit income from trade are rounded.
* Each trade centre gives ~1.15 credits/second base.
* Each 'hop' in the trade route gives *all* trade centres a bonus of ~0.085 credits/sec, even to those which are not directly on the trade route. e.g. if you have 2 adjacent planets, all of your trade centres get an additional 0.085 creds/sec. This scales linearly, so if you have 3 planets in a row you'll get 1.15 + 0.085 + 0.085 to each trade centre.
* Empty systems which are part of the trade route count as hops for the purpose of the bonus (special systems, asteroid belts, etc).
* The route between planets in different star systems appears to count as either 3 or 4 hops for the purposes of the bonus (can't remember off the top of my head - would have to check my figures).
* Distance within a system (i.e. structure placement) has no bearing on the bonus.
* Distance between systems (physical distance) has no bearing, only the number of hops in the trade route.
* TEC can put 11 trade centres on a desert world if they're feeling extra silly. 10 is good because it gives easy numbers for testing purposes (and eliminates some of the errors due to rounding).
* Allegiance bonus doesn't appear to affect income from trade. Neither does planet type (need verification).
* Base trade centres seem to do the same for the two factions I tested (TEC + one other). Probably the same for all of them, with just the techs doing different things?
There are some other factors I haven't checked as I was only going for a rough indication of the trade bonuses, but it's easy enough to set up tests using the galaxy editor (just create a bunch of worlds linked together in a useful way and give yourself a bajillion starting resources). Again, these numbers are from memory so please verify them yourself if you're going to rely on them

. It should give you an indication of how things work though.