Does it make sense to capture more the one from each kind?
For example: When I capture the first Arthron and research it, I will gain 10 % more damage.
What if I capture a second Arthron? Will I have another research with further 10 % or not?
Is there any gain in capturing more then one specimen of each alien?
Without spoiling, there is a gameplay benefit to it after certain research down the line, yes.
Ok, but I won`t benefit from getting the 2nd, 3rd of fourth specimen of the same race, before I got that “certain research”, right?
Or should I try to capture a second Triton or Arthron already? Will it help me?
If you can do it, then capture them. You don’t lose anything unless you have no space left.
This. They’ll take up containment slots, but as long as you have space to spare you might as well stock up until you can use them.
Can anyone tell me if it is possible to See how many containment Slots are full or empty? As some Pandas need more than one Slot it would be good to know if enough slots are available before capturing the Pandas.
You have 50 slots. Small enemies take 1 slot, big enemies more. You have containment tab in personnel.
Yes 
Thank you. Never seen that Info… although iam nearly at the end now.
Can someone please spoil me?
What do I have to research more, to have more benefit from capturing aliens after the first specimen
Align with Disciples of Anu to get access to mutation research. This in turn leads you to be able to build a mutation lab in your bases, and unlocks further research for mutagen harvesting. That’s the research you want. Exchanges your prisoners for mutagen which can be used to mutate your soldiers or build mutogs.
@Lunazathoth:
I got aligned with New Jericho. So do more captives make only sense with Anu?
No, you can try aligning with DoA too (might be easier said than done depending on your current diplomacy levels) or if you don’t want to/don’t think you can, then you can always pilfer their research centres
But yes, without that particular research from DoA’s tech tree there’s no benefit to holding captives other than vivisections and goo/psychic/viral research.
Weird we should be able transform them in food, some look tasty.
I know right? One Scylla could feed my boys and girls for a month!
Incidentally, it always gives me a wry smile that we do acquire food from the Egg research projects lol.
It unlocks the pandoran eggs’ omelette recipe for your operative’s mess hall. 
More spoilery spoilers:
you CAN turn them into food. I don’t know the specific research though, but it’s probably from Anu too.
And a spoilered clarification:
Actually it is a PP tech after the first pandoran vivisection
