Do you have more strength as a pure or some on who has switched jobs?
Strength is reletive to the job you are going into, they are all unique in their own ways, hence theres no such thing as one thats stronger than another per say.

Do you retain abilities, when you switch?
Nope, but when you do your subjob quest at lvl 18, you can sub a job under your main and gain the abilities of that job up to half of its level, providing you actually leveled it up.

I have to be level 30 to become a Paladin or Dark Knight...
Which is better?
You CANNOT compare these two jobs as they are different roles in a party.

DRK, damage dealers, they excel using two handed weapons and using absorb and draining magic spells to attak the enemy. THey usually struggle to get invites due to surpluses and are a damage dealer hence last looked for.

PLD, tank, they are there to take hits, this class excels at using tools at their disposal to keep the monster onto hitting you and taking it. They use Cure spells to keep hate, Flash spell, and other abilities to gain hate, generally speaking in exp parties, subbing WAR is the only way to go because of their special provoke skill that largely generates a spike of hate. They dont do alot of damage as their role is to take hits.

Can Humes be better warriors than Elvaan or Galkas?
Again skill level is what counts, not race sometimes, last night a Taru WAR did extremely well.

Hume can balance everything out so you gotta find something to excel in, Galkas have natural high HP and since they rock with no MP jobs, they can take mores punishment, an Elvaan can dish out dmg but needs more DEX to make it land. This is of course ignoring the skill level of the player. Choose the race that fits you best.

Hume=Never excelling yet never failing, edges out a tiny bit at STR, VIT and CHR
Mithra=About the same as Hume, cept they edge out in DEX and AGI and suffer a bit in CHR
Taru=Excels at MND, INT and magic, suffers in VIT and STR, second best AGI and DEX
Galka=High HP, and VIT, but low MND and INT
Elvaan=High STR, low DEX and INT, also high MND.

Statistically aside its always PLAYER skill that does best, a Taru PLD can do well if they do it right.