Need Expert Advice

I know enough about outdoor fish ponds to be "dangerous". I built three
small ones on my property when I lived in Florida for over 25 years. Now I
am back in New Jersey and I think some of the rules have changed --- not the
least of which is the weather. So here are my brief questions:

1) Some pond suppliers have said I only need go as deep as 18-24 inches for
fish to survive the winter (goldfish and koi --- noting exotic for me). The
pond will only be about 7' x 7'.

2) I never needed a UV filter in Coral Gables but I am seeing that almost
everyone recommends them here. Do I also opt for a pre-filter.

3) In Florida I used the standard 40 mil liner ... should I spring for the
60 mil for added peace of mind?

4) Aside from raccoons (I covered my pond at night --- the only way I could
beat 'em), I never found an adequate way to control frog breeding (tadpoles)
or mosquito larvae. My cichlid pond never had a problem --- it was "snack
food" for them; but my koi and goldfish never quite kept those pests from
intruding all the time. Any ideas?

Thanking you all in advance --- I expect to be a frequent visitor here ---
the threads are very informative.

Norm