Hello, razorrick. Welcome to the forums.
Influence victories are not my forte, but I have enough general knowledge that I think I can help you out.
How do u 'flip' a major or minor race.
First, you can't flip a minor race. To flip a major, you need to have more than 4x the amount of influence that the planet in question is generating. For example, if the planet has 50 influence points, then you will need at least 200 in order to flip it. When you reach that 4x, the game will, each turn, 'roll the dice' to determine whether or not it will flip. The more influence you can exert, the more likely this is to happen. (I assume.) You will know when you are getting close, as a skull and crossbones icon will appear next to the planet.
Does an influence starbase up the ammount of influence for a planet within its radius?
A starbase generates it's own influence, which is added to any other sources.
How many points of influence does an Inf starbase have?
This is determined by the number and type of modules you install. Some modules will give a +5, others can give a +20, or even +30, IIRC. I can't say off the top of my head what the absolute cap is; perhaps someone will come along and answer that for you.
Do planets have a radius of influence like a starbase? It doesn't show one like a starbase when you click on it.
I'm pretty sure that they do, but I honestly couldn't say what precisely that would be. Another one for someone else to answer.
Can spies be used to reduce another races influence?
Absolutely. Place a spy on an influence generating planetary improvement, such as an embassy, and you can see their influence drop.
The AI will vue the use of influence starbases as an act of aggression. It's not about to sit around and allow you to take it's planets. When you begin to deploy these, you will take a diplomatic hit to the relations with that civ. This will continue until eventually, war will be declared. So use these judiciously.
Something else to consider: Influence starbases can be used for more than flipping planets. They can be used simply to increase your area of influence. The Yor's super ablility is determined by their area of influence, so the larger it is, the more valuable that ability becomes. Also, many (if not all, can't remember) of the civs have ship modules that increase the attack or defense values of those ships, but only in your own area of influence. Increasing that makes these more useful.
I hope that this has been of some small help to you. Good luck.