City pond, meandering waterfall; a few questions...

I'm about to build a small pond and waterfall. The waterfall will
begin about 15 feet from the pond, with about a 4' height above the
pond surface. The pond will be approximately 6' x 3' x 2'deep (all
just theory now; please advise). I could have 2 smaller ponds
connected by a "canal" or deep channel, or another short waterfall.
Some fish would be fine, at the very least to eat mosquitos, though my
daughter is urging me to have koi. I live in the city, and my garden
isn't large, so everything is compact. There are definitely raccoons
here in San Francisco, as well as heron, skunks, mosquitos, etc.

So, the questions:

1) I understand that if I have a 2' drop, the raccoons won't go into
the water for fish. How will that work if I have a narrow channel, or
a "canal"? Does anyone have experience with this? I'm imagining the
raccoons wedging themselves between the sides of the canal and
lowering themselves in. Is there a minimum width I need to keep
raccoons out? (I don't want to install the electric anti-raccoon
fence/wire--too unsightly.)

2) Also about raccoons: If I have a 2 or 3 foot deep pond, with some
shallower (6"-18") ledges for plants in water pots to sit on, won't
the raccoons stand on these ledges to fish?

3) I want to install a do-it-myself filter. It sounds like (from
reading around here at rec.ponds) that the waterfall will provide a
fair amount of filtering, since I plan for plants along the length. I
would like the waterfall to be quiet and barely trickle; will that
provide enough gpm for the filtering process, or do I need it to move
faster? Will I need a canister filled with gravel or filter material
as well. Roughly what volume canister should I use? Should the
canister be in the pond, or at the top of the waterfall, or can I put
it somewhere in between, at say, 2' height above the pond? If it were
in between, can I put the pump midway between the pond and the
canister, thus forcing the water through it, then up to the top of the
waterfall?

4) What's the minimum size pond for 2 koi? Frogs? Turtle? Snake?
Are these animals feasible in a small pond?

I've got a rough hole dug, and the path of the waterfall laid out.
I'm excited to get it it! Many thanks in advance for any responses.

--rgesh