As the manual lacks a lot of information, here are some statistics for your convenience.

Weight Capacities:
- Ships 5000.0
- Explorers 7.5
- Soldiers 5.0
- Clergy 5.0
- Natives 7.5
- Horses 130.0

Weights:
- Explorers 10.0
- Soldiers 10.0
- Clergy 10.0
- Natives 10.0
- Horses 80.0

- Food 0.1
- Old World Plants 0.1
- Animals 20.0
- Arms 5.0

- New World Plants 0.1
- Spice 2.0
- Artifacts 2.0
- Gold 1.0

Prices Harbor Master:
- Ships 1000.0

- Explorers 4.0
- Soldiers 40.0
- Clergy 20.0
- Horses 3.0

- Food 1.0
- Old World Plants 0.1
- Animals 2.0
- Arms 2.0

Sales prices in Spain:
- New World Plants 0.2 (= 2.0 per weight unit)
- Spice 0.4 (= 0.2 per weight unit)
- Artifacts 2.5 (= 1.25 per weight unit)
- Gold 1.0 (= 1.0 per weight unit)

Effects of gifts given to clan chiefs:
- Positive numbers have a positive effect on the tribe's attitude.
- Negative numbers have a bad effect.
- 0's have no effect (except loss of gift).

Tribe: Animal / Arms / Food / Horses / Plants
-------------------------------------------------------------
Advanced: 1 / -1 / 0 / 1 / 0
Cultured: 1 / -1 / 0 / 1 / 1
Farming: 0 / -2 / 1 / -1 / 3
Warrior: -1 / 2 / 0 / 1 / -2
Hunting: 3 / 1 / 0 / 0 / -2

Minimum population per settlement type:
Camp: 1
Station: 30
Garrison: 50 (at least 6 soldiers)
Fort: 70 (at least 22 soldiers)
Mission: 150 (at least 10 clerics)
Colony: 301 (at least 5 clerics)

General tips:
- Alt-key repeats the last action chosen with Ctrl. That's helpful to quickly open the map or ask natives while traveling
- Increase DOSBox' cycles for the running game with Ctrl+F12 to make the game ran a bit faster (e.g. to 20000 cycles); Switch to windowed mode (Alt+Enter) to see the current cycles in the titlebar;
- You can increase cycles permanently, by editing dosbox_7cog.conf in the game's folder; Set: cycles=20000
- If you're ambushed, run immediately to the lower-left or left to escape
- To stop these attacks, visit a nearby village, reach the chief while you're attacked and give him a bunch of gifts he likes
- More men in an expedition, yield a wider range of view to chart the map (e.g. 50 explorers offer a good sight radius and are easily supplied with food by trading in villages)
- Always use horse (riding safes food and they offer a lot of additional weight capacity); Leave them at your exploration base at the mainland when returning to Spain (especially in a historic game, where there are only big land masses and few islands)
- Make use of rivers and plains, when traveling through known territories to safe time and food
- Bring as much food from Spain as possible. Settlements will never supply themselves. They use the food you bring and starve if you don't.

A question about trading with natives. If you make the chieftain agree on trading, is there any downside on taking all the items they have to offer and giving nothing on return? I only give them Old World items to make more room for New World items.

A very useful list, indeed.
Thank you very much :)

I don't think so.
I suppose the game sees it as an example of "early colonization contact" where native people would be scammed of their valuables while being offered New World "shiny trash" and alcohol :P