Ps3 limit is 100, when ps4 came out the limit was raised to 1,000 for ps3 and 2,000 for ps4. No one is deleted when you add them but the ps3 gets buggy as you add people past the 100 limit since it wasn't designed to hold so many initially.
If you download the play station app and just add people through your phone, things work much better. The ps3 doesn't delete people but it's still unable to show more than 100 people at once. I do know if you add too many people the ps3 breaks.. heads up?
That's a good question, considering that they extended the total limit of friends after the release of the PS4. Not sure if it applied to PS3, but if anything it shouldn't delete old friends after adding new ones.