First of all, no one, not even the celebrant, has the authority to add anything to the Mass for whatever reason, even if it is the name of “diversity”. Please read what Redemptionis Sacramenutm says:
[6.] For abuses “contribute to the obscuring of the Catholic faith and doctrine concerning this wonderful sacrament”.14 Thus, they also hinder the faithful from “re-living in a certain way the experience of the two disciples of Emmaus: ‘and their eyes were opened, and they recognized Him’”.15 For in the presence of God’s power and divinity16 and the splendor of His goodness, made manifest especially in the Sacrament of the Eucharist, it is fitting that all the faithful should have and put into practice that power of acknowledging God’s majesty that they have received through the saving Passion of the Only-Begotten Son.17
[7.] Not infrequently, abuses are rooted in a false understanding of liberty. Yet God has not granted us in Christ an illusory liberty by which we may do what we wish, but a liberty by which we may do that which is fitting and right.18 This is true not only of precepts coming directly from God, but also of laws promulgated by the Church, with appropriate regard for the nature of each norm. For this reason, all should conform to the ordinances set forth by legitimate ecclesiastical authority.
[8.] It is therefore to be noted with great sadness that “ecumenical initiatives which are well-intentioned, nevertheless indulge at times in Eucharistic practices contrary to the discipline by which the Church expresses her faith”. Yet the Eucharist “is too great a gift to tolerate ambiguity or depreciation”. It is therefore necessary that some things be corrected or more clearly delineated so that in this respect as well “the Eucharist will continue to shine forth in all its radiant mystery”.19
[9.] Finally, abuses are often based on ignorance, in that they involve a rejection of those elements whose deeper meaning is not understood and whose antiquity is not recognized. For “the liturgical prayers, orations and songs are pervaded by the inspiration and impulse” of the Sacred Scriptures themselves, “and it is from these that the actions and signs receive their meaning”.20 As for the visible signs “which the Sacred Liturgy uses in order to signify the invisible divine realities, they have been chosen by Christ or by the Church”.21 Finally, the structures and forms of the sacred celebrations according to each of the Rites of both East and West are in harmony with the practice of the universal Church also as regards practices received universally from apostolic and unbroken tradition,22 which it is the Church’s task to transmit faithfully and carefully to future generations. All these things are wisely safeguarded and protected by the liturgical norms.
[10.] The Church herself has no power over those things which were established by Christ Himself and which constitute an unchangeable part of the Liturgy.23 Indeed, if the bond were to be broken which the Sacraments have with Christ Himself who instituted them, and with the events of the Church’s founding,24 it would not be beneficial to the faithful but rather would do them grave harm. For the Sacred Liturgy is quite intimately connected with principles of doctrine,25 so that the use of unapproved texts and rites necessarily leads either to the attenuation or to the disappearance of that necessary link between the lex orandi and the lex credendi.26
[11.] The Mystery of the Eucharist “is too great for anyone to permit himself to treat it according to his own whim, so that its sacredness and its universal ordering would be obscured”.27 On the contrary, anyone who acts thus by giving free rein to his own inclinations, even if he is a Priest, injures the substantial unity of the Roman Rite, which ought to be vigorously preserved,28 and becomes responsible for actions that are in no way consistent with the hunger and thirst for the living God that is experienced by the people today. Nor do such actions serve authentic pastoral care or proper liturgical renewal; instead, they deprive Christ’s faithful of their patrimony and their heritage. For arbitrary actions are not conducive to true renewal,29 but are detrimental to the right of Christ’s faithful to a liturgical celebration that is an expression of the Church’s life in accordance with her tradition and discipline. In the end, they introduce elements of distortion and disharmony into the very celebration of the Eucharist, which is oriented in its own lofty way and by its very nature to signifying and wondrously bringing about the communion of divine life and the unity of the People of God.30 The result is uncertainty in matters of doctrine, perplexity and scandal on the part of the People of God, and, almost as a necessary consequence, vigorous opposition, all of which greatly confuse and sadden many of Christ’s faithful in this age of ours when Christian life is often particularly difficult on account of the inroads of “secularization” as well.31
Regarding the Creed, Fr. McNamara from Zenit notes that:
Actually, the use of the Apostles’ Creed in the Mass liturgy is the novelty.
Furthermore, the Directory for Masses with Children notes that the Apostles Creed may be used when children make up the majoriity of the faithful at Mass. Majority here does not mean 50% + 1. They need to make up a significant majority at, let’s say, a school Mass. Thus, the default should be the Nicene Creed.