o How big a livewell should a 18-ft boat have? I know this is kind
of an open-ended question. But I am just looking for a "rule of
thumb" kind of thing. Based on discussion from the web, I am under
the impression that a livewell that comes with a boat tends to be
smaller than it should be. I figure that if a livewell that comes
with a 18-ft boat tends to come with a X-gal water tank, I may do well
with a X-plus-20% water tank for the live well.

Depends entirely on the species of fish you plan to put in there. I
freshwater fish, mostly for bass. Most of the freshwater boats I've
fished out of had perfectly adequate livewells for most bass species,
and many other freshwater species, except striped bass and catfish.

o Where is the thru-hull fitting for water-intake installed? Does
it have to be installed all the way near the rear bottom of the boat
in order to extract water when the boat is on plane? I guess it
should. But I am wondering whether this may disrupt the water flow
and make the boat hard to get on plane.

Usuallly through the transom, near the bottom, close to the drain
plug.

o Can the pump suck water into the thru-hull fitting while the boat
is running in high speed? You see I have a hard time visualizing the
water getting sucked into the thru hull fitting when the water is
passing by very fast.

On any planing boat, there will be a void of water right there. When
you install the livewell, best to put in some type of fixture (ball
valve with a remove lever) that will keep the livewell from emptying
when up on plane. Many livewells also have a recirculate feature for
this very reason. Aerates the water already on board when you are
moving at speed, so you don't need to replace water right away.

o There is something called "transom mounted livewell pump". Can it
suck water while the boat is on plane? I cannot see how it will work
because the pipe for the water intake must go below the bottom of the
boat and cause a lot of disruption on the water flow, right?

The transom mounted pumps don't accomplish anything other than mount
the pump directly to the thru hull in the transom. The amount of
suction is not signficantly different than a pump using a hose to
reach the thru-hull fitting. I don't like the transom mounted pumps.
I had one fail at the fitting (stress from the weight of the pump on
the plastic fitting). If I hadn't been there, the boat would have
swamped. Best to have a stainless or bronze fitting through the hull,
with a double-clamped hose leading to the pump, and an above the
waterline loop leading to the tank's aerator.

Thanks in advance for any info.

Hope to read more about your project.