Thank you, Tom - I appreciate the encouragement! And my thanks to all who have liked and thanked. I am trying to write to an audience, but it's not really my style. If you don't mind verbose, please read on. Tom - I would be happy to take you up on the offer at some point; there is a lot of subject intro and solicitation material here that can be removed. I will do the best I can to keep adding to this with my experience and perspective.
But again, I think we could all benefit from the addition of others' experiences and opinions. I know I'm not the greatest vivarium maker in this hobby, and also I know I'm not the only one who has reached some conclusions regarding methods can lead to a rewarding display.
Pendant / Downward Growth
This is an appealing growth habit occasionally seen in vivariums. These plants often grow as epiphytes and tend to extend downward as they grow, which makes them unusual and eye catching. This type includes some Gesneriads, like Columnea, some epiphytic blueberries, epiphytic cacti, and a few Peperomias and Orchids. But like all plants, they have specific light requirements and unless those requirements have been met, they may grow upward - this is something to consider when selecting these plants to be part of a vivarium design. Another thing to consider is the total length the plants will attain. Some may require and tolerate trimming, but there are a few that max out at a more desirable length or grow slowly enough that it can be a while between trims.
So what is the big deal with all these growth habits, why concern ourselves with how different plants grow?
1. Space. There is a finite amount of surface and volume in any given enclosure. Knowing the anticipated size of the plants, how they grow, and where they grow will make planting to achieve a naturalistic scene, that is not overly crowded or awkward, more of a real possibility.
2. Natural Tendencies. Not all of the growth habits occupy the same surfaces, so that allows us to use various types in their niche locations. There are few hard and fast rules about growing plants in a vivarium, but some plants just won't grow far outside how they are found in nature. For example, some epiphytes truly have an aversion to growing in any kind of consistently moist substrate, such as we find in many vivariums, no matter how good the drainage is or how much air movement we add. Similarly, some plants need to grow in some kind of substrate media that remains moist and would never be happy mounted, so they are obvious terrestrials. The lines get blurred when we see a plant climbing a tree; although they appear to be happiest climbing (and that is true), most have roots in substrate in the beginning, so mounting vines to a background with a bit of moist sphagnum to give them an upwards boost may not produce the desired affect, for you or the plant. However, if a small pocket with substrate is included in the background, then 'mounting' the vine becomes a more plausible idea. But again, if the plant wants to start at the ground level, then it may be best for it and the overall look to let it do just that. Placing plants were they want to grow will not only look more natural, but will likely make them happier.
3. Eventual Growth and Maintenance. Seeing photos of how your plants will look as they grow out, if you have not already, is a great idea before letting them go free in a vivarium. This subject alone could inspire pages. Knowing how a plant will grow up, out, over, into, etc. the surroundings and other plants can give you an idea of where they will be happy, but also where the other plants and overall scene will be happiest. This is probably the most pivotal point with respect to creating a more harmonious vivarium. Envisioning how all the plants will grow out can help you find that perfect spot for the plant, add to the naturalistic aspect of the display, compliment other plants, and help you decide if the plant is right for the vivarium at all. If the bromeliad you like grows to 12" around, then you'll need a tank that is wider than that and plan to have a barren spot beneath it or track down some very low light plants. If your Peperomia is going to become a carpet, keep an eye on it and the scissors ready, or plant taller plants around it that can survive the swarm (but watch for over-shading). Unfortunately, there is only so much researching we can do, and sometimes all the photos are misleading, so trial and error comes into play. And plants are like water, they go where they are happy and will surprise you, just another surprise in life. I will say on this matter than the eventual growth of plants has caused me to reduce the amount of fast growing plants I keep around, in favor of slower growing species and those that don't travel far. But you have to walk the line between what you like best and what performs the way you'd like. I have some plants that require regular trimming, and I don't mind that, because I like having them around. Patience, it is not easy to wait for plants to grow in. Fortunately, things can be adjusted as the vivarium develops; you can add, subtract, trim, etc. You can start heavy or light, it is not set in stone
4. Preference. If you really like a particular growth habit and want to see it included, and highlighted, in the display, then choosing the appropriate hardscape becomes important. For example, some shinglers (i.e. Marcgravia) look great growing on the ground level of a tank or up any given background or piece of wood. But to grow them in a way that really showcases the natural beauty of that form, some nearly vertical wood, or (even better) a tree buttress would be the perfect mount. Conversely, if you are planning a tree buttress or vertical surface, plants that desire such a surface to grow upon would serve to highlight it in a natural way.
5. Variety and Interest. A little variety in growth habits goes a long way. Not to say that having all growth habits in one tank would be a bad thing. But it helps if you have the space and appropriate environments for all the types you intend to include. Too little variety can be a little monotonous in my opinion, such as a tank that only includes a vine and a bromeliad. Using a mixture of growth habits adds interest and keeps your eyes moving around all the patterns they form.
6. Quantity. If a plant is going to go nuts eventually, then starting with a smaller piece can help delay that. Some plants may acclimate better as smaller divisions, so starting small can be a good thing. If a plant is a small individual or discrete grower, then multiples can work and help draw some attention to it. This is pushing into another topic all together that can be elaborated on later.
Next will likely need to be how to arrive at planting based on the hardscape design. A short topic, but worth mentioning. Also need to add clumping habit.