As to your first question, you are correct, it is probably a bad idea to use your stardust on weak pokemon as you will catch other Pokemon with a far greater potential than raticates.
Also the fact that you are a low level trainer (assuming this based on your highest raticate being 325cp) means you should hold onto your stardust. This is because as your level increases you will catch higher and higher CP ratatas which can be evolved using only the candies which you have plenty of.
However if you are planning on controlling many gyms right now, then it may be best to evolve as many as you need and power them up as much as you need.
As to your second question, you should evolve your ratatas when you catch a high CP one, then you evolve that and it will be stronger than the ones you have now, since your trainer level is increasing. You can transfer your lower level raticates as you get better ones for an extra candy.