I know it's trying on the patience to get small plants in and wait for them to grow, but, they'll be healthier if you do it that way. A lot of plants don't like to move tanks, so if you pay for really big plants, depending on the plant, they may melt when you plant them in your tank and you'll have paid a lot of money for (now) really small plants. Besides, in my opinion it's hard to plant/scape with plants that are already really tall, with small ones you save money and you kind of get to decide how they grow.
Just my advice, but, save a little money and try to be patient. They'll pay off in the long run. With a lot of stem plants you won't even have to wait that long. Most of your myriophyllums grow very quickly, and grow very tall. I've had them hit 30+" looking great.