Hi there,
First let me say that without details of your flights (where you are leaving from, where do you connect, day, etc.), all I can give you is general info. I used to be a res supervisor for UAL, so I called a friend that still works there to see if anything has changed--overall, not much. United still has positive bag matching, which means that your bags will be on the flight that you are (unless someone screws up.

). Standby is free, providing that you are on a true United flight. As someone posted earlier, some of those 6 flts during that first hour ARE code share with us air. United has a total of 11 flts into MCO the whole day from its' hubs in dulles (IAD) and chicago (ORD). If your connecting city is PHL, CLT, or PIT, you will be flying US Airways. One more thing about connecting cities--they have to stay the same as what you are confirmed on--obviously if you are in PHL, and your confirmed flt is in CLT you're out of luck.
You can fly standby on US but there is a 25/ticket charge. You will need to call US to find the load status on their flights--United won't have that info in their system.
If you want to give me some details, I can give you a couple of suggestions.
Just a couple more things--the airlines will tell you if the flight that you're ticketed on is overbooked. They will not tell you how many seats are left. The seating charts that you see on the web only give you a rough idea as to how full the plane is--all of the seats are not available to be pre-assigned.
Before I moved to FL, I had a lot of success flying standby to MCO on early flts. I always got on the list for the 757 aircraft whenever possible because they had more first class seats than the airbus or 737. More 1st class meant more seats for people to upgrade, freeing up economy seats.