As far as I know there is no problem with adding plants gradually as you get them. Obviously you want to do this with as little disturbance to the fish, but once your tank is planted you are going to have to do some routine maintainance to the plants anyway like removing the odd dead leaf and pruning and replanting the offshoots etc. I tend to do this during water changes, I siphon off 20% of the water and then it is easier to reach everything to add fertilizer tablets under the gravel or trim any dead leaves and move things like new java ferns that have sprouted to where I want them. Then I add the new water.
When setting up a new tank I think most people add the initial plants with the tank about half full - it is hard to see what they will look like unless there is enough water for them to stand up. Good luck, plants really make a tank and the fish do seem to appreciate them.
DD
