You can have the kitten neutered now if you want or by 4 months old. Check with your vet to see if they are ok with neutering now
As far as introductions, the new kitten should be isolated till checked out by the vet for a clean bill of health. Keep him in a room with litter pan, food/water for a few weeks. You can introduce them in a number of ways.
First clip ALL nails so they don't hurt each other. Expect hissing, growling and some possible fighting. Do not interfer unless one is obviously in trouble.
Next try sprinkling both cats with cornstarch baby powder (to smell the same) or use a dab of vanilla extract on their noses when introducing face to face.
Keep the kitten in a room by himself for a few days. Then switch places - let the kitten out and put the resident cat in the room. This way they can check out the other's smell before face to face meeting.
When they seem more comfortable, put the kitten in a carrier and let the older cat approach - this way they both will be safe. If they seem friendly, let the kitten out and supervise their meetings.
When we bought Charlie home (4 months old) he was put in the library by himself. Introductions to the dog were within 2-3 days - they were fine together. Ling (the resident cat) did NOT want to be friends for weeks and months. I think they finally got more friendly after about 2 months of living in the same house.
But Ling is "miss priss" and still doesn't totally accept Charlie and he's a year old now.