Snowdrops starts to root early and are sensitive to drying out. This is probably the reason why it often is recommended to plant them in the green. If you buy bulbs early in the season from a reputable dealer that stores them properly and immediately plant them, then I agree that you should be good. Dried up bulbs late in the season have a much smaller chance for success.
My preference for snowdrops in my own garden is to lift them just as they start to yellow and immediately replant them.
I would personally get a few bulbs each of both N. nivalis and N. elwesii to start with. This will extend the bloom time and you will get a chance to see which one does best for you. Plant in a few different locations to see how they do. Once you are satisfied with their performance you can add more in those areas they do best. Planting a lot of bulbs at once can lead to big disappointments. On my dry sandy soil, they do better in the colder, moister areas. In the lawn, they grow very slowly, your experience may differ.
In all honesty there isn't such a great difference between snowdrops as some pictures will give you the impression of, so buying fancy cultivars isn't worth it for normal garden use. If you want to mass them and hopefully also get them to self seed then you want the species.
Winter aconites do not do so well here for several reasons, however if you plan to mix them with snowdrops, it might be better to plant them in groups, rather tan to do an actual mix if that was what you intended to do. That way you wont risk that one takes over too much. These bulbs directly compete about the same space and resources at the same time, so keep that in mind.
