I recommend you get a good planted book. One of Takashi amano's books would be great. This will get you started, especially with respects to placement. Don't use tall plants in the middle or front, don't use short plants in the back or the middle unless you root it to wood, or rock. Try to keep some contrast, ie red plants mixed into the green plants. I like a good variety, yet still keeping groups of each plant. Other than the basic rules, I place the plants in such a way that they will not detract from one another, so no putting ludwigia glandulosa next to alternanthera rosefolia. Keeping egeria najas away from cabomba can be done, or they can be mixed to give a completely different look of contrast. Ultimately it is up to you where you place the plants, just go with what you like.