Two comments

1) Why not keep the launch rail short and combine with a tethering system that is released on ignition; my own suggestion is based on pinholes at opposite sides of the exhaust tube and a loop of fishing line through these to tether the rocket. The line will be destroyed when the engine ignites, releasing the rocket to slide along the rail.

2) Use a tension sensor (probably a DIY job) on the line holding the balloon to detect when the line goes slack; combine with a timer so that when the line goes slack for a suitable period (5 seconds?) the launch is triggered.