What is the difference (practically) between different types of rennet? I recently started cheesemaking and I purchased some vegetarian rennet from an online retailer. I believe it's microbial (it's a clear liquid, slightly more viscous than water).

When a recipe calls for X amount of calves' rennet (liquid or tabs), I've looked online to try to figure out how this translates into the kind of rennet I have. Online recipes for different cheeses differ - for 2L of milk, a "standard dose" can vary from 1.5mL to 5mL.

Is it possible to add too much (diluted) rennet? Is it better to add too much than too little?
From what I've found it's pretty much a one to one ratio, if you are subbing vegetarian rennet for anmial rennet.  They act simmilarry in equal doses, so you should be fine just following the instructions for calf's rennet.  

I do perfer using calf's rennet over vegetarian, but that may be just be me being anti-vegetarian.