I guess it's a subtle hint that Greed and Greedling both really want friends. For a guy who claims to want everything this world is offering, he can't seriously think that the crew he has represents a serious threat to anybody, even though all the chimeras are pretty tough.
Also, these chimeras are people who probably felt rejected by the rest of society, whether you take the 2003 or Brotherhood versions. So, it was easier for Greed to approach them and win their trust.
I think it has to do with the fact that he is the living incarnation of greed so naturally he wants henchmen, the more the merrier. Plus his chimeras are both human and animal which means he gets both henchmen and hench-animals all in one package, which is perfect for being the living reincarnation of greed.
but if you really want to take a deep more meaningful look into his association with chimeras I'd have to say it has to do with the fact that he wants friends. Like the chimeras Greed/Greedling is not accepted in normal society so naturally he would surround himself with society's outcasts.