Can members of a legion attack each other?
What are all the benefits to building a big legion?

no they cant attack each other as long as they are on your friends list. if you really want to attack, remove them from friends. the benefit is you gain a % bonus to planet defense stats.

And cloaking/attacking

Benefits of a large legion:
1. Passive bonus to all of your planet's att/def/cloak up to +100%
2. Your planets can be guarded by your legion, adding their att/def to the planet and they must be disabled before invading
3. Active bonus to the cloak score to your planet if they are guarding (Can anyone verify this? Or is it only your own cloak that adds.)
4. You can point out ships for them to attack with no rank restriction
5. You can point out planets for them to help weaken
6. They are a deterrent to people looking to attack you (I never target people with large legions, unless they deserve it)
7. Planet sharing
8. Artifact gifting

I've probably missed some, but that's a good start.

@Headless
1. 100% is applied to both planet and guards att/def/cloak for planet. No bonus for individual ships.
9. Sending of artifacts and minerals to help other legions

EDIT: Bad things about big legions with unknown players-> BETRAYAL. You put up an enemy planet for them to weaken, instead they invade it for themselves.