I have a lovely lady customer at work who was going to just get boring old goldfish & ended up getting a heater, a Purple Cambodian Fighter, an Upside Down Cat Fish, a Bristle Nose, some Black Widow Tetra's & Kuhli Loaches all doing wonderfully for the last few months - she now wants to know if a Ghost Knife would be compatible & I have no idea so I told her I'd ask here...
Much appreciated !!

I don't know much about the ghost knives. I know they are pretty timid but can eat really small fish. I'm not sure how compatible those fish are together. Has anyone here had one?

It should do fine with the betta, most times you have to worry about the betta getting the fins ripped off by other fishies and not the other way around.

it depends how big the tank is because betas are very territorial but if it has its space it should just fine but im not quite sure about ghost knives i dont really come a ccros that stuff often.

Ghost Knifes are usually not that aggro, but they do get huge, in my dad's tank there are two ghost knifes, both over Twenty cm now. It is true that they'll eat little fish, the problem is that when they start getting that big all fish start getting little. :D
I really don't understand why everybody worries about the betta, the only problem I've ever had with betta is that all the other fish wants to eat their fins, they are superbly aggressive with each other, but in my experiance they are wimps when it gets to other fish. :P
But on the topic; knifes and ghost knifes get huge.

Waht do you mean by get huge ??? Ghost Knives grow massive anyways...