If you have a horse on trial, I would NOT turn it with others, not worth the chance of something happening.
Just doing that now with a new horse we bought and is going very well.
Horses in pens, where they can smell and interact over strong pipe panels with new horse in his side of the pens.
After a day or two, new horse turned out where it can run around in a larger area, still across from the herd turned out in a now larger area.
After a few days of that, when they take naps standing across the fence from each other, lay down to sleep close to each other, we know they are bonding.
After a few or more days, depending on the new horse, after feeding hay on the ground, more flakes than horses, we open the pens all around and the new horse walks in with the others, the others walk in where he was, they all have the run of all the pens and traps.
Only had real trouble once, a horse didn’t indicate anything under that system, once I opened the gates, he walked in with the herd and then ran at the meekest, softest, low in the herd gelding, grabbed him by the neck and knocked him down and was shaking him.
I got to them and hit him with the manure cart to get him off and put him by himself again and found him another home, where some older ranch horse gave him a better education in horse manners.
He is still there and fits fine, never fought there, but wow, that was scary to watch.
Other than that, new horses find a buddy and get along fine.
Some other problems have been, had one that would not go out to pasture and was guarding the pens, a few that herded some and played keep away with others, but no fighting, which is nice.
Good luck with your introductions.
This is all geldings, with mares or mixed herds, well, that may be a bit different, some just don’t get along as well as most geldings may.