I am no expert on this but i recently adopted a 11month old to two 15 year olds.
What i did was keep the new cat up stairs in his own room and each day for a few days brought him downstairs keeping him in a pet carrier and putting him in the same room as the other cats, just for a few minutes and building the time up each day.(I would do this for about a week in your case!) Also i would let the other cats roam around in his room so they could get his smells.
After a few days i opened the pet carrier and let him out for a few minutes under our supervision for him to roam around.
We did this till we were comfortable with him to be with the others.
You could try doing something like this and see how you get on.
Hopefully someone else will come on here and give you a few more tips that might help.
If all else fails you could keep him in upstairs or away from Shirley until you can find him a home.
It's a shame but poor Shirley to stay out in the rain she must be fed up with him!
Hope this helps.

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