Both GRML and GRFL characters are fully compatible with Unity Mecanim animations, so if Maximo 3D Animations can be used for standard Unity characters "mapped" to Humanoid Rig, these animations could be used for GRML/GRFL as well. You can test this Yourself.
About customization -- what sort of customization You mean? If You ask about morphs -- my characters do not have them. Or You mean something else?