build your own planet
I believe Boris has a point there, rotation will not be affected if there is only gravitational interation between bodies.
However why would we waste a perfectly good planet like Mercury by smashing it into Venus, lots of debries flying around between Mars en Jupiter to do the trick of speeding up Venus (this would indeed mean a steady bombardment of the planet)
Beside if we are able to take Mercury out of its orbit why not place it simply a little further from the Sun, say in one of the Lagrange points of the Earth Sun system, just to make sure nothing nasty comes flying our way.
Besides if we can do all that, why worry about some stray space debries coming our direction ? If we are that good at playing pool I'm sure we will be able to take the perpertrators to a more safe orbit.
Of course all this is just some childs play, if we really want to build ourselves a suitable home we shouldn't make planets for god's sake ! Those stupid balls of rock are just a waste of materials if one considers that a ball provides the minimal amount of surface for a given volume of matter.
An other thing that we are carelessly tossing away is all that nice energy that comes from our sun, our planet gets about 0.000001 % of the energy that the sun emits and we on our turn reflect 2 thirds of that back into space.
The solution : a Dyson sphere around the sun. The sphere would be something like the size of our planets orbit.
We live on the inside and (if by then we haven't found how to generate artificial gravity) a rotation could provide enough gravity for those living along the equator in a bandwith of say a few million kilometers. This would still mean we would have like a million times the amount of surface we have on earth (including the oceans) to play around on (if we could use the enitre sphere it would even be 100 times more)
It would only take the mass of Jupiter to construct something like it which would be something like 100 m thick, so don't go around digging large holes now, you hear !

Of course I am assuming a have developped materials that are strong enough to withstand the pressure and which can be tailor made from simple hydrogen. (fuse away I would say)
We could harness all the energy output of the Sun and while we are add it, perhaps tab some mass out of the Sun so it won't burn up that fast. If we are real careful we might have enough energy for the coming 100 billion years to come.