Was Polyandry allowed prior to Mahabharata?
In general Polyandry was allowed, but it may not be in a "marriage" form (e.g. Draupadi's case). However the women were much more liberal prior to MahAbhArata. Even during Kuru race's time, certain regions were liberal towards women's sexuality. Refer this answer.
"... I want to know whether this story is correct"
Yes. From Adi Parva - SAmbhava Parva:
O thou of eyes like lotus-petals, the present virtuous practice hath been established by that Swetaketu from anger. Hear thou the reason. One day, in the presence of Swetaketu's father a Brahmana came and catching Swetaketu's mother by the hand, told her, 'Let us go.' Beholding his mother seized by the hand and taken away apparently by force, the son was greatly moved by wrath. Seeing his son indignant, Uddalaka addressed him and said, 'Be not angry. O son! This is the practice sanctioned by antiquity. The women of all orders in this world are free, O son; men in this matter, as regards their respective orders, act as kine.' The Rishi's son, Swetaketu, however, disapproved of the usage and established in the world the present practice as regards men and women. It hath been heard by us, O thou of great virtue, that the existing practice dates from that period among human beings but not among beings of other classes. Accordingly, since the establishment of the present usage, it is sinful for women not to adhere to their husbands.
"were there any other examples of Polyandry prior to this story."
Yes. From the same chapter, PAndu states below examples:
O princess, who is devoted to her husband, it hath also been said by those acquainted with the rules of morality that a wife, when her monthly season cometh, must ever seek her husband, though at other times she deserveth liberty. ... Thus, O timid one, was the existing usage established of old by Swetaketu, the son of Uddalaka, in defiance of antiquity. O thou of taper thighs, it hath also been heard by us that Madayanti, the wife of Saudasa, commanded by her husband to raise offspring went unto Rishi Vasishtha. And on going in unto him, the handsome Madayanti obtained a son named Asmaka. She did this, moved by the desire of doing good to her husband. O thou of lotus-eyes, thou knowest, O timid girl, how we ourselves, for the perpetuation of the Kuru race, were begotten by Krishna-Dwaipayana.
and also from Adi Parva - Vaivahika Parva
Yudhishthira then spoke, saying, 'My tongue never uttereth an untruth and my heart never inclineth to what is sinful. When my heart approveth of it, it can never be sinful. I have heard in the Purana that a lady of name Jatila, the foremost of all virtuous women belonging to the race of Gotama had married seven Rishis. So also an ascetic's daughter, born of a tree, had in former times united herself in marriage with ten brothers all bearing the same name of Prachetas and who were all of souls exalted by asceticism.
" who were Rishi Shwetaketu and Rishi Ullapad. Have they did any major work."
Yes. They must be indeed of great merit. The substantial info I could find in the same page is:
It hath been heard by us that there was a great Rishi of the name of Uddalaka, who had a son named Swetaketu who also was an ascetic of merit.
Other references are also found, like Vana Parva - Tirtha YAtra Parva:
the sacred hermitage of Swetaketu, son of Uddalaka, whose fame as an expert in the sacred mantras is so widely spread on earth. This hermitage is graced with cocoanut trees. Here Swetaketu beheld the goddess Saraswati in her human shape, and spake unto her, saying, 'May I be endowed with the gift of speech!" In that yuga, Swetaketu, the son of Uddalaka, and Ashtavakra, the son of Kahoda, who stood to each other in the relation of uncle and nephew, were the best of those conversant with the sacred lore ...
Related: How many husbands are permitted to a woman (polyandry) in ancient history?
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