So, if your tank is a standard 110 gallon, it has a 48" x 18" footprint. Which means it is 48" long and 18" from front to back.
A water cow has no business in a tank that size, as they can hit 18 inches +/-. This is one of those fish which I seriously recommend nothing smaller than a 180 gallon. They're ambush predators and need a diet of meaty foods. Incidentally, for what it's worth, I hate the name water cow. They don't chew cud and there's nothing cow-like about them. I prefer bicolor sleeper goby, but I know water cow is the common name most widely used.
I suggest you skip a pleco altogether, also. A flowerhorn will be hard on a pleco at best and kill it at worst. If you insist, absolutely stay away from a bristlenose as it probably won't last long at all due to it's smaller size and relatively thinner armor. I also wouldn't waste a lot of money on something fancy since the flowerhorn will probably beat on it, tatter the fins and cause scarring, if not kill it, and at best won't be so fancy anymore. Which leaves a "common" pleco. Also subject to murder by the flowerhorn. And there isn't just one species that is sold in the hobby as a common pleco. "Common" pleco refers to a number of Hypostomus and Pterygoplichthys species which can max out at anywhere from 10" to 24", or even a bit more. At best you'll learn to recognize and identify a handful of different species so you can rule out what a particular pleco is not. In this part of the country, Pterygoplichthys pardalis is the species I see most commonly sold as "common" pleco. This species is actually fairly easy to ID, and they are tanks that can stand up to a lot of abuse. But they also hit 18", and are very long-lived. My first one was over 16 inches long and lived close to 20 years in my 225 gallon tank. It was one of the first fish I ever bought. The one I have currently is just shy of 15" long and right around 11 or 12 years old. A 110 gallon is too small for this species, a lot of fish shops won't take a pleco that size back, and contrary to popular belief public zoos or aquariums won't just take any large fish offered them.
A single flowerhorn will be fine. Just be prepared to keep one fish and one fish alone. That can be hard for someone new to the hobby. Also, if you end up with a nice male, the nuchal hump (the big round bump on the forehead) will be subject to damage by rough, sharp or pointed edges on decor, so the tank will have to be without any kind of decoration. BTW, the nuchal hump is also called a kok when discussed by serious flowerhorn breeders and hobbyists.
So after reading this, I hope you will seriously reconsider your plans, if it isn't too late already.
WYite