Originally Posted by Flibbity Floid
So long as they appear on the map, yes. You can complete them anytime they're available. You can also replay them in the start menu.
There are really only going to be 4 "Mission" types on the map which will be the T, F, M and ?. Once you start the ?, I believe it changes to the character icon.
From what I recall, the best ways to manipulate the stock market in your favor is through the Assassination missions for Franklin. I'm not entirely sure how it's done but it has to do with the companies/names involved and obviously how you play the market. I didn't do this though.
Cool. The ? you refer too, is that another person you are keeping secret or the freaks ones? Exciting!