I have had a large group of up to 14 piggies over several years and currently have got four groups between 3-7 piggies.
If you are rehoming privately, you may have to consider a quarantine and a vet check unless the new girls have had this at the shelter. (Are you in the US?)
As to introductions: Please keep the girls next to each other for a few days if possible, so they can get their bearings and get to know each other through the bars. That will take a lot of extra stress out of the introductions proper and there is less of a likelihood of them overreacting due to overload/fear-aggression.
Whether they will gel, depends to a large extent on how to the two top sows can sort out the dominance issue between them, as one of them will have to step down and may not do so willingly. The undersows will usually go along with the flow and just fit in as long as their leader does, but they will pick up any tensions from her. Each bonding is different, as it very much depends on the pigsonalities involved.
Please plan a whole weekend for the bonding. Do the intros on neutral ground that is not part of any group's regular territory. Don't have any hideys or stuff in with them. I prefer either a plate of fresh grass or a heap of hay on some newspaper in the middle, so they can meet over eating together.
Once initial acceptance has happened (or not, in which case you will have to abort), the hierarchy sort-out will start. It generally happens between the two group leaders and then slowly works its way down the ladder until all piggies have found their new place. The dominance phase usually takes around two weeks, but it can be quicker or longer. If tension/aggression levels are persistently high, but just below the line of actual tussles, I would separate for the night and try again the second day, still on neutral ground, but it is best to get at least a few hours under the belt on the first day so the very worst of the dominance is over and things can be worked out in a more relaxed frame.
Move your new group to the rearranged and completely deep cleaned cage when things have been settled for a while and major face-offs are over. Please leave out any hideys with just one exit for the first few days, so no girl can be caught in a tight corner during a chase, and the typical throwing out of premium places can't cause problems with nips turning into actual bloody grazes etc. If necessary, cut two sides off some carboard boxes to use as open shelters.
Here are more information and tips. Please read them carefully and ask any questions you may have:
https://www.theguineapigforum.co.uk/threads/introducing-and-re-introducing-guinea-pigs.38562/
https://www.theguineapigforum.co.uk/threads/dominance-behaviours-in-guinea-pigs.28949/
https://www.theguineapigforum.co.uk/threads/sow-behaviour.38561/
https://www.theguineapigforum.co.uk/threads/importance-of-quarantine.108034/
