Not with gravity.
Accretion assures the planet that it will neither be hollow or flat. Small rocks can be shaped other than round/oblate but something as big as a planet will be shaped, filled and round/oblate by the force of gravity.
(Unless we have aliens out there that can shape planets! )
Hollow, yes..............flat, no.
I highly doubt it.
The gravity of each seems to indicate that they are all solid rocks (true, the centre could be molten rock, but rock just the same)
No, a planet is a celestial body that has accreted enough material, and thus mass, for gravity to form it into a spheroid. Even most, if not all micrometeors that have been observed are spheroids in shape and not flat.
Sorry, I did not address the question about a planet being hollow. Since many planets are formed from small solid material, sand and grain size matter and up to meteroids and possibly asteroids later, many are solid throughout. While this is true of rocky or known solid planets it is not yet certain about gaseous planets. It is not yet known if gas giant planets have a gaseous, liquid or solid core.
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