Agreed, the fish mentioned are only suggested to sometimes eat flatworms, and it seems to me that it is not really something to rely on. Some individuals might, but most won't. If it was the only fish in the tank, and you had lots of LR to cruise in and out of, I don't think a sixline would be that poor of a choice simply as a fish stocking question. But definitely not, IMO, with another fish in there. Personally, I would stick with one fish and look into more inverts. I have one damsel in my 15g and he would kill anything else put in there, fishwise.
Also, sixlines are far less picky about diet than mandarins and scooters. Very few mandarins or scooters take to frozen brine at all, and usually it requires training.