I disagree on the cats and loaches, as to 'it doesn't matter what substrate', but I will say I've kept them on gravel and sand and it can work with either. I think it's less about the substrate and more about cleaning - with gravel there can be tons of crud you don't see down between the gravel, whereas with sand it often sits on top and is visible so you are more prone to clean off the crud = cleaner surface for the bottom dwellers to forage around in. Kuhli loaches for example can certainly hide in the gravel at Petsmart, and do, but I would argue it's not 'best' for them. Besides, it's pretty cool to watch a horseface or skunk loach explode out of the sand when you were sure nothing was in that spot of the tank.
Depending on the quality of the gravel and what kind of fish you have (e.g. fish that dig a lot vs. fish that ignore the substrate), the color will leach or get scraped off over time and that nice pretty gravel will look ugly in time. But if you're OK replacing it periodically to make it look nice, it's certainly your call. Make your family happy...it will go a long way if you decide to set up future tanks.
