Whilst life is never 100%, it is generally easy to mix pigs of different ages, and he should integrate with them fairly easily.
A few points to consider
1. Pigs need a "flight distance" - that is when two pigs meet, they will push shoulder to shoulder to see who is stonger. When they do this, the losing one needs to be able to run away. Generally you need a minimum of about 30 ft. When the losing one realises that he/she will not win, then they run, persued by the other, the winner will give up quickly if the loser continues to run. If the loser hasn't the distance to run, then the winner will continue to harrass, and the loser continue to fight as he/she cannot run away. Wth your mix, it may well be the boar who approaches the girls.
2. You may get no agression at all, or you may get a few scratches on the shoulders, and any "fighting" is likely to be short lived. Have an anti-bacterial spray such as septi-cleanse handly to just spray any scratches.
3. They may not sleep togther on night 1. A second shelter should be provided to allow for this.
4. Introduce when there is still some hours of daylight left, so that new one finds boundaries, water etc. in daylight.