In the interest of not giving the same information to people on Twitter all day every day, here is another cheatsheet. This time listing the interaction between Grid and other layout methods, when you make something laid out using one method a grid item.
Grid acts on the container
The key thing to remember with CSS Grid Layout is that it acts on the container. You define tracks and these constrain the items inside. Most other methods, including flexbox when used as a “grid”, act on the items. The only exception to this being multiple-column layout, where the column-* properties are defined on the container.
If you float something, if you create a grid with flexbox, if you use inline-block, or display: table these items stop behaving with the properties that they have from that layout method as soon as they become a grid item. What you do need to watch out for are the widths you have applied to make those legacy grids work. Setting widths back to auto within a feature query checking for grid support, will be required for many layouts.