First off, WELCOME!!!
This is a GREAT PLACE with AWESOME PEOPLE!!! I would have given up on planted tanks long ago if it wasn't for the help I received here!!!
That is some AWESOME WOOD and ROCKS!!!! They will make a BEAUTIFUL hard scape! I too usually place my heater next to the intake of the canister filter, and it has always worked out well! Do you plan on using a temp controller for the 2 heaters? The reason I am asking is that it is very difficult to set 2 heaters to the exact same on and off point, and usually 1 of the heaters runs most of the time while the other does nothing. Then the overworked heater fails quickly, and can either just stop or the worst case scenario is it will stick on and cook your tank! I just read a article about this exact thing, but I can't remember where I read it! I will try to find it and let you know where to find it! So my advice would to be get 1 good heater of the proper size and save the smaller heaters for other tanks! My FAVORITE heater of all time is the Otto Computherm!!! They are hard to find, heck I don't even know if they are still being made or not, and somewhat expensive, but they work!!! The one I have now has been going for over 8 years and holds the temp +/- .2* of where it is set! It has a external thermostat with a temp probe that mounts in the tank with the heater. I got them at my Friends fish shop, but he has closed it down, so I don't know where to get them! Anyway I just hate to see people use cheap heaters and wind up loosing their fish and plants! I have done it myself, so lesson learned here!!
On you r substrate if you ask 10 different people you will get at least 10 different answers! in my experience I have used Flourite, and while ok at first I had problems with BBA growing on it as it aged. So now it is in my flower beds at the front of the house waiting to get tilled into the soil this spring!!!
I "dirted" my 55g this time around, and it has been growing plants very well! I used Miracle Grow Organic Choice Potting Mix (Orange Bag) covered by Black Beauty "sand" blasting mix. The Black Beauty is actually Coal Slag, but it is non toxic and cheap!!! $7 for a 50# bag at Tractor Supply! My only complaint about it is there is a bit of a oily film that you will see when you are rinsing it, and you can't rinse it all the way off, but it goes away in a few days after being in the aquarium from filtering and aeration! There are MANY people that have used it here and to my knowledge no one has had any problems! My tank was "dirted" in October IIRC, and it is growing great guns, and all of my fish are very happy! Even my Albino Corys who have spawned 3 times now!!!
I plan on making some root balls to add a bit of ferts to the dirt though, as I have noticed some of my crypts have holes in their leaves, but the rest of my plants LOVE the dirt! However if you are one who likes to move things around allot dirt tanks might not be for you, because it can get messy when uprooting plants that have been there for a while! You will pull some of the dirt up in the roots, and that can cause problems! BUT you will pull up root tabs sometimes too if you use them, so there are problems with all substrates! You just have to figure out what you like best and try it! If it doesn't work out you can always change it later on!
Your lighting choice should be good, though you will need to watch your photo period length if you are not using CO2 of some kind! 4- T8 bulbs can still put out pretty good light, so you will need to watch out for algae!
You will probably need a little bit more circulation too, but that is a easy fix with all of the different type of power heads that are available! You just want to make sure there are no dead spots that can lead to algae!
I can't wait to see how you get it all setup! I love 75g tanks!!! You are off to a good start too, so keep us in the loop, and we will help ya however we can!
Best of luck to Ya!