The laterite goes down first, then the flourite, then cap it all off with the rest. You want to be sure that the laterite is buried a minimum of 3 inches, personally I dont like the stuff in my tanks, it works great but if you like to move things around as much as me it is near impossible not to uproot the stuff into the water column.
2 bags of flourite for a tank that size is really not much but if cost is a factor , which at those prices is, then 2 bags is better then none.
I have seen beautiful tanks grown in nothing but sand and gravel but you must remember to keep the gravel very small in size and keep the substrate fertile for the plants if they are root feeders.
Flourish Tabs, Root Tabs, Jobes Plant sticks etc. will do the trick , but again, dont use coarse gravel or the nutrients can easily seep into the water column... sand works great for this.
Good luck with your new tank !