your heater can be placed in sump as well as the skimmer. depending on the
type of system your going with (reef, FOWLR, or FO) will tell if you need a
"power filter" or not. reef's depend on the live rock and sand bed for
natural filtration, a skimmer helps reduce the need for water changes to
keep water quality in check. a sump can also consist of a "wet/dry"
(recommended in FO (fish only) setups)

lights................the watts per gallon is a really bad thing to ever get
your mindset on. wattage is best kept to "depth of tank, and type of light"
with surface area playing a partial roll in this as well.
on my 75 I have 800w of MH, it was a reef and done excellent with all
corals/clams. its now a predator tank, and other then sucking up all that
power and $$$ its still great.

the 4x96 would be in my opinion the better choice for a 75 gallon tank. I
have 4x65 pc over my 29 gallon reef and its not enough for higher light
demanding corals and clams. my seabe anemone is nicely colored and growing
but to be honest it would do much better with more lighting. 4x65 PC just
isn't enough and surely wouldn't be enough over a 75 except for softies
(shrooms, zoos and such).

with hard corals I honestly don't think you can have to much light, they
just have to be acclimated to it slowly. softies on the other hand can get
burnt/blistered quite badly even with very slow acclimation.

the different lengths of bulbs are mostly different mfg's not necessarily
for fitting in tighter spots, but it is a good side effect of the shorter
bulbs.

as for your question on the cover glass, I personally don't use one on
anything except HQI bulbs which require a UV protective shielding. any
glass/acrylic will decrease the intensity of your lighting and is IMO not
needed.

hope to have helped
kc