I can think of a few different ways to mark a sequence to help you put videos in the 45 and 15 second slots.
One way would be to simply use markers.
Another way would be to create a 45 second color matte with the name “45 seconds” and a second matte with the name “15 seconds”. Put those on track one. You can copy/paste the two mattes over and over until you get to the 15 minute, 30 minute, or however long you want your standard video to be. Select both of them, Ctrl-C and then Ctrl-V and you can do this quickly.
You could also think about putting your intro and outro on track one as well.
After that, simply put all of your videos on track two and adjust them to the correct length. Although I would probably set the correct length in the source window before dragging them to the sequence.
If you want something that stands out more, make the two mattes different colors and set the sequence to show Continuous Video Thumbnails, and expand track one a bit so you see the color of the 45 second matte and the color of the 15 second matte.