How to display the voucher box only for a specific client group

[prstshp_joe]

Hi,
 
There's already a topic with the same title that dates back to 2014, so I decided to open up my own thread.
 
So far this is what I did (and works), but I wonder if this is the right way (version 1.6.0.11).
 
In "\themes\My-Theme\shopping-cart.tpl" I changed all 3 lines with "{if $voucherAllowed}" to "{if $voucherAllowed && !in_array($defaultGroupId,array(5,6,7))}"
 
I added in "classes\controller\FrontController.php" in line 372:
"'defaultGroupId' => Customer::getDefaultGroupId(Context::getContext()->customer->id),"
So I would know what the GroupID of the currently logged user is.
 
Fiddling with FrontController.php is not something I feel comfortable with, is there a better way?

 

Now I'm stuck on changing the checkout process so the first step out of five is to log in. Any ideas?
 
Thank you!

Edited April 12, 2016 by prstshp_joe (see edit history)