In my opinion, putting all of your eggs in one basket as you are describing isn't a great idea. If I were to use micro instances for hosting I would certainly launch a dedicated instance for the mail server and then a second LAMP instance for everything else. I would even go so far as to launch a third instance just for the databases depending on how many and how large they are.
You will be happier in the long run when you decide that you have to either do some maintenance or upgrade to a larger instance and you can migrate in stages instead of all at once. You will also be happy that if someone hacks any of your servers, only a portion of your setup will be down instead of everything until you can get it fixed. That doesn't make for happy customers.
Why not setup a dedicated mail server instead?
In my opinion, putting all of your eggs in one basket as you are describing isn't a great idea. If I were to use micro instances for hosting I would certainly launch a dedicated instance for the mail server and then a second LAMP instance for everything else. I would even go so far as to launch a third instance just for the databases depending on how many and how large they are.
You will be happier in the long run when you decide that you have to either do some maintenance or upgrade to a larger instance and you can migrate in stages instead of all at once. You will also be happy that if someone hacks any of your servers, only a portion of your setup will be down instead of everything until you can get it fixed. That doesn't make for happy customers.