Is this a standalone? Rather not have to read the other comics.

Moderately so. Cursory knowledge of the shows should be all that's needed.

So is Tommy's Son a new SPD Ranger?

The main comic is already Tommy focused, is this really necessary?

There were fan rumors that one of the original SPD rangers from the show was a kid of Tommy's. Though no clue if they ever outright said which. I watched more Deka than SPD, so I only know about the fan speculation one was.

Someone more versed in SPD would have to answer that.

He sells. The Green Ranger of the original MMPR is the character that nearly everyone wanted to be as kids because he had the coolest zord and costume. Which translates to better sales now because of that emotional attachment to who wore the suit everyone wanted. This is why Green, White, and Gold of the original seasons stand out the most. But in Tommy's case, he was both Green and White.