Regarding multiple start positions, another technique is to take advantage of how JJ2 assigns warp targets. This information stays for the whole game (it doesn't reset with each level in other words).

So, what you do is you put a warp at the end-of-level area, and another warp at the start position in the next level. Then tweak the warp targets to get it to send you to a specific point. The problem is it can sometimes be hard to get the game to assign warp targets properly, and you also have to count how many you have in the level. This system has the advantage of not requiring you to place the warp targets in the level that you are exiting from (which would be problematic if you are going from a smaller level to a bigger one). I posted more detailed illustration of this a few years ago, but I'm too lazy to dig it up.

I like the method Troglobite posted a bit more, since a bit easier to use, as long as the level sizes are right. It could use a bit of refining though. For example, if you copter while hitting the exit, the system won't work. Putting ceiling springs over the exits will solve this by forcing the player to stomp. This means it takes up 4 tiles instead of 3, but that's not a big deal. I also recommend replacing the collapsing scenery with a sucker tube set to wait time 4, speed 0. Also, you still need the spring if you decide to go with a tube, again due to coptering.