As many times before, I agree with Fleshy. Fleshy and I seem to agree on alot of stuff and somehow he always beats me to these threads.
When starting out a planted tank it's not that big of a oops if you don't have Co2. It's just a little harder to balance out your photo period and algae growth.
As for your 2 questsions.
Yes it is possible to inject to much co2 into your tank. The plants are mostly photosynthisizing the co2 when the lights are on, Although they continue at night in a small forum(without getting into some pretty advanced botnay we will leave it @ this) Therefore the Co2 your injecting all throughtout the night will cause your PH to drop, and most importantly the oxygen gets used up quite fast at night for plants cellular respiration(Botany is a little rusty) causing your oxygen supply in your tank to decrease causing the "Gasping for Air in fish" This heavily affects your PPM on your co2 - If your aiming for 30Ppm - I would strongly recommend a solenoid to stop the co2 at night. I would think anything below 15ppm will be fine in a 75G tank.
Choosing a filter for your 75G - I'm not sure how much you know about planted tanks so I"ll spill a little more beans. I would strongly suggest a canister filter (Although you don't have much choice on a 75g) In a planted tank you want to minimize the off gasing of your co2 during the day. This is done mainly by surface agitation. For example a HOB Filter or Air stone. It's fine to have an Air Stone running during the night and this would off set the co2 being on in the night and your fish would have a sufficent supply of oxygen through the night(This is a option if you don't want to turn the co2 off during the night, just a little wasteful)
15W - Is 1.5WWP I'm assuming your light is somewhere in the 8000k Range which is a little high for plants I would try to aim for a 6700k especialyl if your only pumping 15W. But with the right conditions I've grown "High Light" plants in Low light Setups without a problem you'll jsut expirence slower growth.
When you setup your 75G Buy as MANY plants as you can afford. It's a common myth for planted tanks to buy one plant here and one there and plant and see if they survive but unfortunetly that's not the way it should be done. This will help you "Silent Cycle" your tank, and your plants will out compete the algae for nutients.
I also recommed looking into how to prep your plants from a petstore or where ever you buy them from. The biggest headache you can imagine is hitch hikers coming into your nicely planted aquarium from the LFS or another member. This can be done with a Bleach dip, Hydro Peroxide Dip and a few others.
It's really a hit or miss deal with the balancing of your Photo Period, Nutrients and Water quality. Remember when in doubt water change and you can never ask to many questions.
I hope i've set you in the right direction and again me agreeing with Fleshy that your on the right track and off to a great start. Good luck and any questions / concerns can be directed at me through PM.