A higher temp will increase their metabolism which can help them grow faster - as long as you feed them accordingly. The faster metabolism usually also means the fish will have a shorter lifespan (how much shorter I don't know). 84 is pretty high to keep them at. They would probably survive just fine, but it really is unnecessary and a waste of electricity/money. Also, raising the temp in the tank is one part of treating for ich. By keeping the tank at an elevated level you increase your risk for getting a resistant strain of ich.
Also, I have never personally kept demasoni, but most people say to keep them in groups of at least 12 - unless you know that your group has WAY more females than males. Demasoni may be small, but they are known to wipe out tanks overnight as they reach sexual maturity. Not trying to scare you, just want to give you a heads up. Since you already have 14 fish in the tank, adding another 10 demasoni may be a bit much - but if you have the filtration and the willingness to keep up with necessary maintenance it shouldn't be too big a problem since demasoni do max out around 3-4" (I think).
Also you might want to think about adding some more rock, especially if you end up adding more demasoni.
Otherwise your tank is looking great!!
