There is only a warrant for singing the psalms in public worship. It undermines the argument for scriptural warrant if we simply say that we sing scripture in order to be safe. The other difficulty is that the 67 paraphrases are of course very loose and half produced by hymn writes such as Doddridge and Watts. There are also 5 hymns approved together with the paraphrases which do not even attempt to be renditions of scripture passages. ttp://www.ccel.org/ccel/anonymous/scotpsalter.toc.html
The doxology was given up by the Scottish Church at the time of the Westminster Assembly for the reasons of uniformity and that there was no scripture warrant for singing this even though it was in the words of Scripture. The General Assembly's Act of 1650, which adopts the 1650 psalter excluded the use of other compositions and doxologies.
The following is from The Pattern On The Mount: Being An Essay On Purity Of Worship In Opposition To Recent Innovations by Walter Scott, 1877.
http://easyweb.easynet.co.uk/~jbeggsoc/Scott.pdf
The Paraphrases had thelr origin during the dark days of Moderatism in the Established Church. The first collectlon of Paraphrases was published in 1745. lt was remitted by the General Assembly of the Church to the various presbyteries, after which it came to be used in worship. In 1775, a committee was appointed to
revise that collection, and, after being considerably altered it was again published and transmitted for the consideration of presbyteries on 1st June 1781 with a declaration allowing it "to be used in public worship in congregations where the minister finds it for edification." It was only partially adopted at this time but it
gradually came into general use throughout the church. It has continued in the Free Church and is also used in the United Prcsbyterian Church by permission of the Synod.The objections which militate against hymns apply equally to Paraphrases. In some respects they are more dangerous than hymns. Unlike hymns they profess to be a paraphrase or translation in verse of passages of Scripture, while in many cases, entirely misrepresenting the meaning of the sacred text. But what device has not been tried whereby to get something
of men into the ordinances of God? When the evil one cannot get man to give up the worship of God he does the next best by getting them to corrupt it. What God appoints is an ornament, hath beauty, is for glory, but let man set up ought in the worship of God, it hath no beauty but blackness, no holiness but iniquity and
God must be worshipped in the beauty of holiness. (1.Chron. 16:29) (Greenhlll on Ezekiel).