New young male-two older "brothers"?

so this younder male cat has adopted us.. and we already have two other males -one is 2 1/2 the other 1 1/2.. both are neutered. the new male is not. The new cat is NOT aggressive at all,very mellow. the now "middle" cat hisses at the new one and won't let it near him. the baby just stares and not retaliate or respond. the oldest one will sniff the baby, but is much more easy going. The oldest has been around many hyper dogs so he gets along well w/ most others. the middle-not nearly as much. the middle is my trouble child (he has learned he can climb a certain tree to get ON the house!) the older ones now will not sleep on my bed bc the baby has been there. is this bc the baby is intact?.. the baby may be about 7-9 months-not a year.. rather small. if we get the baby neutered will this change- we plan on it.... but we havent' the funds right now.. maybe a couple weeks..