Generally, yes - if there are a few different ways of writing something (like, common ways), it's fine - as long as you don't denote force as Z, mass as G, and acceleration as T, or something ridiculous, it should be fine. I think the important thing is that you are clear what each letter means (include a box on the side with "F: force, m: mass, a: acceleration" or something like that). It's probably okay to do whatever you like as long as it's clear what it is - you could do pokeballs if you really wanted to.