Negative battery terminal sparking on contact

First guess is either the two wires at the starter are jumpered together, or there is something wrong internally with the starter, or you somehow positioned the main batt cable on the starter so that it is shorting

A very long time ago I learned that "if you have a problem" after you have "done something" the problem is very likely related to "whatever you just did"

Yeah thats probably some good advice haha. The mini starters have the terminals facing inwards towards the engine block so it was difficult to tell their positioning. I'll be taking another look, thanks