You can find other threads that deals with sub jobs for ranger. Pretty much the acceptable subjobs for rng are war and nin; and sam if you know what you are doing.
Rdm has nothing that can really benefit a rng main. I wouldn't do rdm/rng either because you won't have any access to good equipments that makes ranged attack worthwhile. Rng sub is pretty much good hunting. If you really really want to sub rng for combat, do sam/rng. I think rdm ppl recommend subbing blm. If you want a rdm main, do blm so you can nuke with spells with high reliablilty.
Red mage sub won't add much to a ranger. The En-weapon spells won't work with ranged weapons and your debuffs won't be as good as a red/white/black mages. As there'll always be a mage in the party later on they will have no use. You don't want to be throwing around cures either, you already get WAY to much hate for that.
Ranger sub for a red mage won't help either, outside wide scan for farming. The one big problem is that when you have a bow out you can't do anything else. Unlike melee if the bows out and someone needs a heal you won't be able to heal them. While red mage sub doesn't really add anything to a ranger a ranger sub on a red mage can get someone killed.