All the canons pertaining to the reception of converts clearly teach that admittance to Holy Communion can only take place AFTER the individual is officially recieved into the Church, never before.
Furthermore in the decree of the 7th Ecumenical council when certain unlawful practises were introduced the Fathers of that council stated:
Those, therefore who dare to think or teach otherwise, or as wicked heretics to spurn the traditions of the Church and to invent some novelty, or else to reject some of those things which the Church hath received (e.g., the Book of the Gospels, or the image of the cross, or the pictorial icons, or the holy reliques of a martyr), or evilly and sharply to devise anything subversive of the lawful traditions of the Catholic Church or to turn to common uses the sacred vessels or the venerable monasteries, if they be Bishops or Clerics, we command that they be deposed; if religious or laics, that they be cut off from communion.
In the 8th proceeding of that same council this canon was added:
" If anyone breaks any ecclesiastical tradition, written or unwritten, let him be anathema"