On the liquid pre-op diet but feel like I am gaining weight

I went to walmart to pick a few more things up before I have my surgery, 1 week away.  
I was looking at the stool softners and had no idea which one I should get.  I know that any of them I could take now but I wanted to get something that I can use after surgery too.  Any suggestions and when should I take it after I have surgery?

Colace works really well.  That's what I buy at Kaiser.  My gallbadder surgeon gave me that for post-op so I just kept using the same thing.  I didn't use it right after VSG.  I think I was told I could use benefiber about two weeks post op.  We have some nurses around here that might have good advice, but the best is to ask your surgeon what is recommended for you.