How to handle different data type for same property for a HTTP or REST API - web-services

Question

I work on a web application using AngularJS for the frontend and the backend for Symfony2. I want to make a web service so I can have a one page app. I am wondering about how I should handle the data exchange between the two layers.
Consider this use case:
A user wants to create a project in the application and must specify whether it be as a customer or supplier in that project. The creator of the project gives the project a name and a description. Then he enters the email address of the contact person (who will be the customer or supplier in the project depending on the choice of the user who created the project). When creating the project the system checks whether the contact person is already registered otherwise it sends an invitation to join the application and waits. It is not necessary to invite the contact person at the creation of a project that can be done later.
I imagine the API is as follows:
POST http://app.com/api/projects
Request:
{
"name": "My Project"
"descrption": "My Project'description"
"vendor": {
"user_id": 3245
}
"client": "myclient#email.com"
}
Response:
{
"name": "My Project"
"description": "My Project'description"
"vendor": {
"user_id": 3245,
"name": "John Doe"
}
"client": {
"user_id": 2754,
"name": "Peter Doe"
}
"creator": {
"user_id": 3245,
"name": "John Doe"
}
}
So we can see that the type of data exchanged in the creation of a project differs.
Can I link directly to my model application (after validation of course)?
How should I go about updating a project?
Is it better to create Transfer Objects and managed into my business logic and a TransfertObject for input and one for output?