I have 2 of them too, and recommend the black ghost knifes.
I have one about 10-12cm's in a 4ft tank and 15-16cm's in a 5 ft tank they are so cool - but they don't come out during the day usually only in the evening or when food is dropped in.
I just have them in with a community group of fish like a large kissing gourami, angels and gold gourami and a few others with no problem. *touch wood* now that I have said that.
and also no problems with plants either. I did have two smallish angels with no eyes one day and have since read they like tasting tiny angel's eyes.