A kitten as a friend for your 18 month old male cat?
If you do decide to get a kitten for a companion for your cat you need to slowly introduce the two together. They may not like each other at first.
Is your cat(the older one) a male? If so he is doing this because of jealousy and/or spite. Male cats do not usually attack kittens. Just like people the kitten(child) may have done something to upset the cat making it hiss. Then later the cat forges the kitten for whatever it is the kitten may have done, hence the nicer show of behaviours.
Yes, a six-month old kitten is very likely to have reached sexual maturity and able to breed. This is why it is recommended to have the cat neutered as soon as possible. If the kitten is healthy and weighs enough, it may be neutered as young as two months old, although the usual age for this procedure is four to six months.