Awakening the Rinnegan To awaken the Rinnegan, can you just get any Uchiha's and Senju's chakra, mix them together and get the Rinnegan? Or do you need Ashura's and Indra's in particular (i.e. their reincarnation's)?  

Do you still need to go through some very sad or aggreviating situation to awaken it, or do you automatically get it the moment you mix the two chakras together?
To get the rinnegan you need to have the mixture of the Ashura and Indra chakras. As Uchihas are descendants of some of them and the Senju of the others, the mixture of these also allows obtaining it. The clearest example is Madara who uses the chakra of the first Hokage to obtain it.

You do not need any more requirements, but it is not a trivial process Madara itself took more than 10 years to wake up the rinnengan.

SPOILER ALERT


  Sasuke has a special rinnegan is different and more powerful, this rinnegan is called Magatama Rinnegan. He obtained it by receiving the moon crescent, I think dark, side of Hagaromo's (the Sage of Six Paths) chakra.