Hey mate, Welcome to LOTRO

Guards get more AoE threat skills later on, at the level cap a good guard can hold a shit load of mobs even with the whole group beating on everything.

RKs are amazing single target healers, so if only the tank needs heals then the RK would be best for a small group. Plus they can dish out the DPS when you dont need the heals.

No focus target in lotro, so its hard to tank and burn down different mobs than what the tank is currently selecting.

If its your 1st time boxing lotro i would suggest to run a group composed of all the same class, probs RKs or Hunters and just level and muck around with everything so you can get a feel for how it all works. Then when you start your mixed group you have a bit of knowledge on certain things. Mixed groups are pretty tough boxing in lotro.

Also what server are you playing on?