How to introduce new horse to elderly herd

[CazD]

I'm thinking of taking a 13 year old horse on loan but wondering how would be best to introduce her to my current herd, who are all elderly. Current herd comprises the herd leader who is 21 and has arthritis in her front legs, 24 year old semi retired pony and 30 year old retired small pony. My biggest worry is them all galloping around together when they are first introduced as, apart from kick injuries, the herd leader goes v lame if she runs around, especially as the ground is hard at the moment. I don't want to cause her any damage! I'm also nervous of anything happening to the loan horse, as she obviously isn't mine.

What would be the easiest way to introduce them?

 

[JLG]

This may not be practical but can you fence off an area for the new horse to go into for 7-10 days so that all the squealing can be done at a distance?
You also have to accept that there may be a few shennanigans when the newbie actually goes into the field but if they have had the chance to see each other without direct contact for a week then this is usually minimised to a bit of squealing and stamping. Do you know how your newbie reacts in a herd situation - is she likely to be stroppy with the leader? Is the herd leader still in work - you could do some short hacks with the 2 of them as well so you are controlling their interactions.
Have over reach boots and brushing boots on everyone if possible for a couple of days once they are all in together - I assume that no-one is shod behind? If your loan horse has shoes on it might be an idea to at least take the back ones off until things settle. My advice is based on geldings however, so mares may be a different ball game altogether. It is nerve-wracking and to some extent you just have to let them be horses. Also is there a Plan B if the newbie doesn't fit in?
Fingers crossed for you.