Koi aren't that easy to sex. You look for the width of the body when they are adults. This typically takes 2-3 years. Unlike goldfish, koi wait to breed.
As for breeding your own koi for you pond... you'll need a pond for that. You have to grow out the koi to over a foot long which isn't possible in a 55 gal tank. Koi are bred in ponds as they need the seasonal queues to know when to breed. It will be less expensive to put those 5 koi in a 1000+ gal pond and then buy another five butterfly koi to grow out in your 55 gal than to set up a pond just for breeding.