How do you get other force legends?

[jedink]

I am using Wei and i go to a stage that has the other forces card in it and i find the box but it just has a random material in it

do you have to be on that force to get the legends, if so then how did i get liu bei and a few other forces on my wei side?

[Axl DS]

You generally have to either:
1) In single player, be on that person's force
2) In multi-player, have someone on that force select the stage

The only exceptions to this rule is anyone who can be found on Chapter 6 and unaligned characters.