Yes, prune them back - these trees are best with a little and often approach really, rather than a hard prune, otherwise excessive growth can be stimulated. As for the yellow leaves, do you mean actual yellow leaves or a yellowish deposit on the leaves? If its just a few which have yellowed, nothing much to worry about, though water shortage might have caused it. Needs a couple of gallons a week in dry spells for its first 2 years. If its a yellowy orange deposit, that's a different matter.

Thanks for your help. The leaves are yellow, but now it seems to be picking up again, so I'm keeping my fingers crossed.