As far as I am concerned, unless you are breeding to show, breeding to the SOP to preserve the breed (which means you just keep one breed, no rainbow mix of colors), or you just want to, there is no reason to worry about keeping breeds pure if you are breeding them yourself. Do as you want about that. Just my opinion.
As far as white colored birds, I really like the Delaware, but that is just personal preference. I've been happy with the ones I got but my goals might be different than yours. There are plenty of white birds out there that would do you quite well. One reason I mention the Delaware is that I don't see any barred birds in your current flock. The Delaware has barring. You don't see that in the Delaware except in the black feathers, but that may be a way to introduce barring into your flock. You would need to save a rooster from the Delaware to do that since a hen only gives barring to her sons.
As far as which rooster, I'd suggest a red or buff one if you want to keep a color variety in the chicks. If you get a black rooster all your chicks are probably going to be black in the first generation. White is trickier. There are three different genes that can give you white and you don't always know which genes the rooster has. If the white rooster has black feathers in the tail and hackles, the white is probably coming from Silver, but an all-white bird is either dominant white or recessive white. You usually don't know which and they have different reactions in the color for the first generation. The white may totally disappear in the first generation or it may show up in strange places, like you may get a red chicken with a white tail. It just depends on what other genes are present from the hens.
With your mix of hens, if you save a rooster from the chicks that is a cross, not a purebred, there are a lot of options as to what colors and patterns his chicks will have, especially if you mix him with other crossbreed chicks. If you know he is a cross with other colors in the mix, it does not matter if he is black or white. He could still father red or buff chicks. To me, that is one of the fun things about hatching chicks from a barnyard mix. You never know what color down the chicks will have or what color or pattern the adults will show.