By default,
AoS doesn't really have any army building rules (anything goes, as long as your opponent agrees). However, there is a separate book called the General's Handbook which has point values for the units, as well as more restrictions on what units you can take.
If by units moving individually you are talking about movement trays,
AoS doesn't use them. As long as the figures end their movement within 1" of another figure in its unit, you can put them in any formation you want.
AoS technically doesn't care about the base, whether it is square or round, since measuring is done from the model - but the models do have a standard base size. Chaos Warriors use 32mm round bases now.
AoS can be played with as few as one model per side, but it might be a bit simple. I think a good size to aim for would be one hero and two or three units - roughly the size of the
Start Collecting! boxes. There is a smaller starter box called
Storm of Sigmar which has a really tiny number of units, but I'm not certain how it plays.