I'll respectfully disagree that you *NEED* to have a variety of prints and solids and sizes of prints to have a successful scrappy quilt. Even the degree of contrast can be negotiable. You do need different values to achieve some pattern, but those values do not have to be super high contrast -- it does help to have the amount of contrast somewhat consistent throughout, though. So you can run into trouble if you have some blocks where the contrast is jet black vs. snow white, and then other blocks in the same quilt are cream vs. tan. You can pull off a scrappy quilt with lower contrasts if you are careful and controlled about keeping the range of values very purposeful. Taking a photo of the blocks on a design wall and then looking at them in grayscale is a quick and easy way to check how well you're doing.
I think that the rule that the smaller your pieces, the more inclusive you can be for a successful scrappy quilt is probably the best guiding principal. If you only see an inch or two of the fabric, chances are it can work as long as you are able to scatter the more compelling colors or patterns relatively evenly throughout. For me, vivid reds, yellows and some oranges are almost always going to need some careful placement. Sometimes a neon or bright grass green acts the same way too.