Are there any fast moving Cichlid that can be added to the tank?
No. All neotropic cichlids are fairly quiet fish, movement wise.
Edit: Just thought about a Tiger Shark or a dwarf puffer, are these an option with the Cherry Barbs and Zebra Danios?
No. Here you are in to very different parameters (puffer, all but one are brackish) plus puffers tend to be feisty (nippy), depending. And they are slow fish, not active, so bad on that score too. Tiger Shark, I need to know the species, I cannot remember the genus. But I do remember reading that this is not a home aquarium fish.
Second Edit: How about Corydoras or odessa barbs, both seem to be fast moving and need to be kept in a school of about four?
Corydoras must have minimum five, but this is a highly social fish that will always be in signifcantly better health if it has many more. I have 50 in my 70g, representing 12 species, and they are thriving and spawning all over the pace. I've had some of them for over 9 years, though that is nothing for this fish which can live much longer--up to 20 years has been documented in aquaria, in the right environment. In your 75g/288 liter, I would have 12-15. Several of each species is best, but I have found they do seem to not care too much just so long there are lots of them. In my tank, some species spend more time together, others rarely. Corys level of activity is a bit different from upper water fish. Cories are "active" but I would describe it more as bumbling along the substrate, over wood, plant leaves, etc. Always on the move, but not hyperactive. Cories are frequently maintained in tanks with shoals of discus, or angelfish, or they are fine with gourami. "Active" may have different aspects.
For hyperactive, maybe Odessa barbs; they need a group, minimum six but a few more is again better. This is a very active swimmer. Would work with other similar active upper fish, but again not if you decide on sedate fish in the tank.
Another thing I thought about was having a section of the tan where the substrate is sand instead of round, smooth gravel and stocking about 3-4 Kuhilis.
This rarely works, and in my view never really works. Water will move around naturally (regardless of filter, but that is a factor too) and fish disturb the substrate. Unless there is some sort of permanent divider this rarely works. The other thing is appearance; this is not "natural" and it immediately not only looks artificial but it physically lessens the space, making the tank look smaller.
Last edit I promise.... Can anyone vouch for the reliability of the website aqadvisor.com? Also could you guys take a look at the list I made there and lemme know what you think?
This site and a couple others are sort of "OK" provided one keeps in mind that no site can ever be accurate. There are factors that simply cannot be programmed into an auto program, they take human thought intervention. We have been talking about some of these in this thread, but there are several considerations for a community tank (community being more than one fish species): water paramters (GH, KH, pH, temperature), aquascape (substrate material, wood, rock, plants--any of these may be essential to this or that species so they must be factored in; simple example, having a pleco in a tank without real wood means a sick fish and premature death because wood is an essential part of the digestive system), water movement from the filter (some fish must have strong currents, some fish totally the opposite and need a swamp type movement), lighting (most fish need less than what we often provide, but for some this is essential), tank size obviously, numbers in the group for a shoaling species, behaviours of each species, mature size...and probably others I can't remember just now.
Looking at the data on AA, I would up the numbers of zebra danio (9-12) and harlequin rasbora (12-15). This rasbora is more quiet than the danios, but it is a bit more "hardy" whatever that means.
Byron.