The heater should help a lot--fish are cold blooded animals, so their entire metabolism is reliant on heat from outside sources to help push it along. Does the heater have an adjustable temperature dial? I can't tell from the picture, but adjustable heaters are the only design that I recommend because they have a reliable thermostat and they are a higher quality product. Pre-set and other non-adjustable designs can underheat or overheat the water and don't give the user any stability or control. If the heater is adjustable, I recommend keeping the dial on a low setting to start with and slowly adjusting it upward toward 78 degrees over the course of several hours.
As far as the pleco and the water conditions are concerned, OFL is right, the pleco is not a good match for this kind of setup. You should rehome it using local resources such as craigslist, your local aquarium society, or simply return him to the pet store if you don't have a larger tank to keep him in. As far as the tetras go, I would maintain the same number of them until your filter is safely cycled again. Once the cycle is complete, you can get a few tetras at a time until you have a full school of individuals.
I highly recommend getting a good liquid master test kit--these are indispensable for maintaining proper conditions in your tank, especially since you just annihilated any bacteria that may have been living in your tank with the Maracyn treatment--so you have to slowly cultivate again all the bacteria you lost. Going through the Nitrogen Cycle with fish in the tank can be tricky, you should take the time to research and understand the process and then purchase a good liquid master test kit, such as this one: http://www.fosterandsmithaquatics.com/product/prod_display.cfm?pcatid=4454
During the cycling process you will want to make sure that the ammonia level and/or nitrite level in the tank is hovering around .25ppm, so that the bacteria have a food source and so the fish in the tank won't be harmed too much by the ammonia level. You will have to do frequent partial water changes and test often to maintain these levels while the cycling process is going on--this process normally takes around 3 weeks to complete depending on temperature, aeration, and ammonia levels. Once the process is complete, though, you will only have to do one weekly partial water change--probably only 30% or so.
Live plants could potentially help you out quite a bit, they use the waste your fish produce as fertilizer and as they consume it, they promote good water quality. You want to look for very specific species that thrive in low light, such as java moss, java ferns, anubias, anacharis, bacopa, etc. Some people also use bog plants like pothos for these purposes--simply place a cutting in the water so that any leaves and most of the stem sticks out into the air. This plant should flourish as long as only the roots are in the water. However, you will broad spectrum (fluorescent) light in order to sustain any aquatic plants you add. The basic 18 watt light strip is more than enough for many plants, and they aren't expensive. Here's an idea of what you might consider getting:
All-glass canopies are cheap and very light--they also allow additional ambient light to pass through to feed your plants and illuminate your tank. If your office has fluorescent lighting, this is a great and inexpensive hood option. It will also prevent your betta from jumping ship--they are notoriously good jumpers.
A simple 18 watt light strip will be able to feed your plants and illuminate your tank without causing dramatic temperature fluctuations. Incandescent lighting can stress out your fish, because when the light is turned off and on, the temperature changes significantly--fluorescent lights use less energy and give off a fraction of the heat of incandescents.
I recommend purchasing any items you want to buy online--the prices are much more reasonable and the variety is much greater than any pet store. Flat rate shipping on Foster and Smith is very reasonable, as long as you order a few items at once, you tend to save a significant amount of money.