I think it was good for me to get multiple breeds when I got my first hatchery chicks. I couldn't decide on which breeds, so I read about behaviour, size, egg color, and ability to forage for food. After awhile I decided what breeds suited my setup and now I pretty much know what I like. Unfortunately, I got too many chickens. Several times I have given away extra chicks, hens, and young roosters to lessen my flock. Personally, I would read the descriptions of some of the larger hatcheries (Meyer Hatchery in Ohio, Murray McMurray in Iowa, Cackle and Ideal in Texas) and find out what you like. Smaller hatcheries have more of the common breeds--Mt. Healthy near you is convenient and you could just pick them up which is much easier on the chicks. Don't have any shipped too early in the season because of cold stress, and be sure to order early (before Christmas) so that you will get what you want before the rarer breeds are sold out. Local feed stores are also good for starting out with a few chicks at a time.
It is good to have variety, it doesn't hurt anything, I don't personally have any idea why I like mixed flocks I think I just like the different colours, sizes, and feathering. Just having one breed is kind of boring to me! I think I really just comes down to personal opinion in the end though!
I have a mixed flock, Started with one one breed and have "branched out". My last two orders were from Meyer, and have one due next week also, they are a good hatchery. Good luck with the new chicks. Make sure you check the hatch dates on the chicks you want. know right now they do have some limited availability.