When I first converted my 75g to a planted tank, I had the larger type gravel also. I followed some recommendations to add some laterite to my tank and it was a disaster! The water was continuously cloudy for almost two months!
I finally gave up the laterite idea and switched out the substrate to Eco Complete and Tahitian Moon sand.
If you want to add the laterite, I would suggest you add a good amount of sand to your tank to keep the laterite from leeching up into the water column. It is very unsightly to have an orange cloud in your tank all of the time, trust me
You may want to check into some Schultz Aquatic Plant Soil (SAPS). It is a brownish red color that looks pretty good once it is in the tank. It is light weight, so you may want to mix this with some sand also. It can be found at Wal Mart and I'm sure some other places that cater to the pond crowd. It is fairly cheap if you can find the 20lb bags. It has a decent iron content (can be used to replace the laterite) and an excellent CEC. You should be able to find silica sand at a better LFS for pretty cheap also.
I have read that play sand is a bit too fine but have used it in the past (mixed with SAPS) without a problem. Some say it tends to compact after it has been in the tank for a while.
Hope this helps