The ```:``` syntax should only be used when you close the php block inside the function.
So that only happens when you output html.

So yes, you SHOULD mix the 2, as they have different uses.
Use the ```:``` syntax for:
```<php if($foo == $bar) : ?>
   <p>String</p>
<?php endif; ?>```
And use the normal ```{``` syntax for:
```<php
if($foo == $bar) {
   $foo = 'bar';
}
?>```