Is it possible to create multiple submission forms for one content type? I want to allow my users to submit different type of content using different types of templates (like in tumblr.com where you get a specific template/form for each content)   

At first, I was thinking about creating custom content types, but this is a workaround because I dont really want to make different content types (all nodes are blog posts). I just want to create different forms for the content submission.

What is the proper "Drupal way" of doing that?
The proper way to do this in Drupal is using multiple content types, but if you want (for whatever reason) to still stick to your solution, you can write a custom module, and modify that one before rendering.

Here's an example using the default page content type and hiding the body. Use the Devel module, and dsm() to inspect the form.

function mymodule_menu() {
  $items['mymodule/add'] = array(
    'title'=&gt; 'mymodule add',
    'description' =&gt; 'add content',
    'page callback'=&gt;'mymodule_form',
    'access arguments' =&gt; array('add content'),
    'type'=&gt;MENU_NORMAL_ITEM,
  );

  return $items;
}

function mymodule_form() {
    global $user;
    $node = new stdClass();
    $node-&gt;uid = $user-&gt;uid;
    $node-&gt;type = 'page';
    node_object_prepare($node);

    module_load_include('inc', 'node', 'node.pages'); 
    $form = drupal_get_form("page_node_form", $node);
    hide($form['body']);
    dsm($form);
    return drupal_render($form);
}