I would go with aquariumplants.com "MILLER IRON CHELATE" if you're worried about any type of Iron deficiency in the plants, or you can go with pure laterite, or some sort of laterite.
I'm slowly going with a 40 gallon MTS myself, and I'm going with both laterite, and adding a bit of Iron Chelate to the Dolomite/Muriate of Potash mix.
Were you able to find Dolomitic compound for the calcium/magnesium in the tank? If not I know a very cheap site to get it from. Just let me know.
15g Fluval Flex - Will be a planted shrimp tank