Question: Assume there was a super advanced alien race that for some reason, wanted to add a new planet, of lets say 1 Earth mass, into the Solar System. Where in the inner Solar System *(The part of the solar system inside Jupiter's orbit, so asteroid belt is included)* could we put this planet, so that it would have a stable orbit for billions of years?

*Note: I would preferably not want have this planet orbit closer to the sun than Mercury, and would like for it to affect the inner planets as little as possible, although I care more about the first condition rather than the second.*

*And also, the planet needn't be habitable*

Answer: 
Make Mars a double planet. Or Venus, or of you do not care about Earth much, replace the Moon with the planet. The previous stability should remain more or less the same, apart from some disturbances that might or might not manifest in the long term (as long as the resonances remain stable, you do not have much to worry about).
The planet would be able to be the most stable in a orbit between earth and Mars because that the biggest gap between notable celestial bodies of the inner solar system. So gravitation pull of any bodies would be most minimal hence being able to keep a steady orbit as long as the sun lives.

Edit:I apologize by I messed up(dyslexic). It rather crowded between mars and the belt you best bet would be between earth and mars.

Another solution would to have an orbit perpendicular of all the other orbit since most bodies in this system tend to stay on one plane.