An Insane, but Interesting Idea: Fleet Launched Orbital Craft

I could very easy point you to problem: to fast pump large volume of liquid, need high pressure, and in this case will need fast connector for high pressure.

If you will look at Russian R-7 rocket, it have specific "package" design even without propellant crossfeed, but just to start all engines on ground.

And connector for typical rocket pressures of 60Atmospheres or more is really hard to do.