First of all I really really appreciate you Khusi for the question you made.👏
Being in 21st century it is really hard to feel the time when polygamy existed. There may be so many different viewpoints regarding the issue but one thing I am sure about in NO CONDITION a man or woman CAN BE HAPPY with his or her spouse with multiple partners.
You asked Can wives in a polygamous marriage be friends? My answer is NO.
1. It is human nature to feel insecure when not getting desired attention from the partner. Even sibling rivalry has the same logic (only here the attention is from parents).
2. There will always be a fear to be neglected and lose power or control over the husband which is no good for her own existence.
3. In some parts of India there was a practice when a man from certain caste could marry as many as he desired and there were a lot of men too who had multiple wives and the wives stayed at their parents' home and the husband visited them when he got time. I have read that there were so many men who actually visited their wives once in a year (because of the number of wife they had). There were so many women who almost forgot their husbands' face. Just think about the situation. Still they retained their faith in marriage.
4. Here we are talking about the situation where the multiple wives stay together. For me this is worse. In the above mentioned example the wives generally did not have to face one another (exception was always there). Now when they are all in a same place obviously there is rivalry, jealousy, possessiveness due to insecurity and all. All want to grab attention and time from the husband and there no friendship can exist I believe.
5. Now the situation becomes the worst when we talk about an emperor and his unnumbered wives. From what we have read it can be said that an emperor would have the least time for his wives considering the responsibility he had. Earlier it was also natural that only a few women had their own identity and they were literally lost in the crowd (royal crowd of course!) They got married, entered into a new life. Spent a few dreamy days and then all daydreaming only. Think about a harem full of ladies ... how can we expect Rukayia to be friend with Jodha (not supporting Ruk's actions though). Honestly each single lady from the harem must have wanted to get control over others and a few won.
6. One exception. If there is a situation where the husband is absent or not paying attention to any of his wives, some of them might get closer to each other but again, sharing romantic thoughts with a woman who also nurtured the same feeling for the same person ... the least desirable I must say.