yes

If you have rookies to convert into experts, you will boost your true ability more by
adding experts and not vets, because as sby stated above, 20 vets = 80 experts, better get
200 experts no ?
but thats at short term, in the long run the vets will be more useful, especially if you
already have 80/90 vets.

Correct. You can keep on adding experts until you run out of rookies.

Ok, but if I have for example 600 experts on a sailor, and 50 recruits, I only can get
50 experts more for that sailor?

The amount of vets and experts that you can have on any one sailor will depend on
what level & class the sailor is. Again, I would suggest that you view the link below. It
will be able to explain everything you could want to know about NF & its sailor system.

http://nf-guides.com/sailors.htm

1 more question, what is the maximum amount of experts that I can add, I have
now like 200 experts on my sailor, am I still able to pump another 100 experts on
it?

los veteranos siempre dan mas % ke los expertos pero creo ke deverias escojer el
pak ke mas se asemeje atus necesidades y no loke te digan, pues cada uno dira
una cosa distinta , suerte

Well lets look at it this way...

According to NF-guides.com vets are worth 4x experts. So 20 vets would = 80
experts.

The choices you have are...

200 Experts
20 Vets (aka 80 experts)

The stats say you should lean towards the expert pack. Although adding 200 experts
or 20 vets to a sailor will not make huge difference in the overall sailor ability. I will
say that if you are already near the 100 mark for vets, then even though experts will
offer a better stat gain, it might be best to get the vet pack. After all you can burn up
a lot of experts trying to get from 80 to 100 or from 99 to 109. These are just my
thoughts on the question.

Please note that you can buy an Elite sailor out of the 3000 points. And elite engy or
repair could be a very nice option as well.

The best choice depends on how many experts/vets the crew already has.

As for how they work, Adalbert's guide covers this rather well:
http://www.navyfield.com/board/view.asp?Num=84358&Sort=D07

Asking for opinion on a forum to make a choice is the worse thing ever.
Check what you currently need so you can decide.