Replacing the power steering pump on a friends 97 F150. New pump came with a rubber O ring and teflon O ring in a little bag. Also in the bag was instructions on how to fit the teflon O ring but nothing about the rubber O ring. Manual procedures only mention the install of a new teflon gasket. I can't figure out for the life of me what this rubber O ring is for...?

We got the job done last night, all went well other than breaking the pulley removal/install tool. No leaks..

I hate having something left over and wondering where it goes.

Also, the new pump came with power steering fluid in the pump... I'm guessing the right thing to do was to remove the power steering fluid, prime/fill it with mercon trans fluid and continue the install...but, at some point the decision was made to leave it and top off with trans fluid after the install was done (not by me).
Is the mixture of the 2 fluids going to be an issue (Ive read some say the 2 fluids are similar and wont be an issue, but have also read it can be a long term problem) or should the whole system be drained and atf used to re-fill.