It is generally a good idea to keep things alphabetical, unless there is a good reason to do otherwise.

With matrices, it can be okay because an $n\times m$ matrix is really a linear transformation from $\mathbb{F}^m\to \mathbb{F}^n$ (where $\mathbb{F}$ is your field of definition), and so viewed this way, the $m$-dimensional space comes before the $n$-dimensional space.

So I think you are okay sticking with what you have.