"It is widely held that Peter paid a first visit to Rome after he had been miraculously liberated from the prison in Jerusalem; that, by "another place", Luke meant Rome, but omitted the name for special reasons. It is not impossible that Peter made a missionary journey to Rome about this time (after 42 A.D.), but such a journey cannot be established with certainty. At any rate, we cannot appeal in support of this theory to the chronological notices in Eusebius and Jerome, since, although these notices extend back to the chronicles of the third century, they are not old traditions, but the result of calculations on the basis of episcopal lists. Into the Roman list of bishops dating from the second century, there was introduced in the third century (as we learn from Eusebius and the "Chronograph of 354") the notice of a twenty-five years' pontificate for St. Peter, but we are unable to trace its origin. This entry consequently affords no ground for the hypothesis of a first visit by St. Peter to Rome after his liberation from prison (about 42). We can therefore admit only the possibility of such an early visit to the capital."
http://www.newadvent.org/cathen/11744a.htm {Peter, Prince of the Apostles - Catholic Encylopedia}
No --- Not a contradiction between Justin Martyr and the Catholic Encyclopedia on this matter, but an admission by the Catholic Encyclopedia that Eusebius's, Jerome's, and all subsequent pontifications of a 25 year Roman Bishopric for Saint Peter is without foundation.
So let us summarize as to where we are at this point:
1] There is no evidence in Scripture, or the writings of the Ante-Nicene Fathers, or historians of that period for a Roman Bishopric of Simon Peter of any length of time.
2] But there is evidence in the writings of the Ante-Nicene Fathers [Justin Martyr . . . ] for a 25 year Roman Bishopric of Simon the Magician whose followers were called Christians.
Is that a fair summary thus far? Do you have any evidence to the contrary? If so, present it.
If not, then should we now turn to the writings of other Ante-Nicene Fathers to see what else they can add to Justin Martyr's statements regarding that 25 year Roman Bishopric of Simon Magus whose followers were called Christians?
The Encyclopedia is being cautious and states that thechronological notices in Eusebius and Jerome are calculations rather than direct evidence. It does not say that the hypothesis is without foundation, but merely that the notices offer no ground for the hypothesis.
there is evidence in the writings of the Ante-Nicene Fathers [Justin Martyr . . . ] for a 25 year Roman Bishopric of Simon the Magician whose followers were called Christians
There is, more accurately, evidence of an influential blasphemous teacher by that name, in Rome about that time, with whom St. Peter had earlier had a dispute in Samaria.