I've never played FFXI so I can't really comment on that except to say that it's my understanding that mobs respawn WAY faster in WoW than in that. So while it sucks to have your camp cleared on you, the mobs will be back shortly.
As for the rest - I'm not exactly sure what's going on. Are the other people killing stuff you're not actively fighting? If so - again, an annoyance but like I said above, if there are a lot of people clearing stuff fast then the respawns are generally equally as fast. Another option there would be to whipser that person (type /w playername) or just in say and ask if they want to group instead of fighting for the mobs.
If someone is attacking mobs after you've started attacking them then that's a little different. If you're pulling multiple mobs and only being able to loot one then that's where some game mechanics comes in (this only applies to other players helping you, not NPCs):
If you've done damage to a mob (just applying a debuff doesn't count) then that mobs is tagged as yours and you should be able to loot it once it's dead no matter how many people helped.
If you didn't do any damage to it (usually when I pull multiple on purpose I try to make sure I damage them all) then whoever does damage to it first gets credit.
It could be that the paladin was trying to "help" (which is odd because particularly on my paladin I always found it REALLY annoying when people "helped" me) but didn't know you didn't have all the mobs tagged.
Best thing in a lot of these situations is just whisper or ask them what the heck they're doing. Sometimes you get a rude reply or no reply (in which case, probably easier to just move elsewhere), sometimes you get a group and sometimes you just work around each other a little better.
I hope this helps, and welcome to WoW!