Autoresponder query.

So I have already taken $5 monthly subscription . Here are the consolidated questions

I just created a test campaign. I created a new autoresponder. I created a new custom field pickedDate. I added the following rule

Based On:PickedDate

Send: After 0 Days 00:00 GMT

Type: One-Off

I then went and added contact and set the pickedDate to today's date. I was expecting the mail to go immediately but the mails has not yet arrived. What am I doing wrong. Basically I am trying to do what should happen after a signup of my services. How will autoresponder be triggered if I am manually adding my contact and setting up my custom field pickedDate to a date.

Is there a API for Zoho Campaigns. Can I trigger an autoresponder by firing an API from my code.
if I take $5 monthly subscription how many mails can I send to 500 subscribers