The Hitchhicker's Guide to the Galaxy Method
To be entirely clear, this is the process of building planets in something like a shipyard and then "shipping" the planet to its desired location.
This is really infeasible, despite Mag Making the planet and then moving it into place is really energy intensive. Here are some questions which may illuminate why:
- Where are you going to get materials strong enough to pull a whole planet along?
- Are you going to make giant rockets on one side and move it into place that way? If so, what are the fuel sources for your rockets? How do you place them so that they don't just drill into the ground or deform your planet?
- What does the fuel need to combust or otherwise propel? Does this material come from your planet?
This will only work if you have a cheap and effective way of moving the whole planet into position. This problem alone is too large a one for most technologies, even if properly scaled up, to do. You simply need super materials and magic rockets to do so. You're better off trying to find Magrathea with your infinite improbability drive, or ask Slartiblartfast, wherever he is currently.
The "Death Star" Process
This is more likely. You can get a massive asteroid, or several smaller asteroids, and move it/them into position. You can get the correct distance from the sun, the correct speed for you planet, etc. You then send more asteroids or other items to you planet. Eventually, your planet grows to be pluto sized, then moon sized, and so on until you have your planet.
After a while, you may not even need to move that material "gently" into position. You can just send a comet/asteroid of material and have it impact your proto-planet! Talk about reducing costs!
You will have to do some tricky things to maintain angular momentum. This is so that your planet, with increasing mass, does not slow down and potentially crash into the other one. This is easily avoidable with the "death star" process.
The down side of the "death star" process are, of course, any rebels, ewoks, or other random things getting in the way. I suggest marketing it to the galactic empire as a real estate project, even if you never intend to settle people there.
Gravitational Effects of a New Planet
...is actually pretty small. The Solar System is a big, big place. If you do some research into tides, you'll note that the moon is what mostly affects them. All the other planets and even the sun, despite their enormity, don't actually contribute all that much. Unless you build your new planet to be massive, or really close, your other planets shouldn't feel all that much.