James, sorry it took a while to respond. Let's try to figure it out.
First of all it's better to have Free role, you can simply disable all actions for this role and create several roles in addition with the necessary settings.
As far as I have understood you have difficulties with the relationship between Roles feature and User Credits plugin. Could you please describe in more details the way your site is configured and how it should be configured to achieve your needs. I will be able to find a possible solution for you basing on the provided information.
Also, please check my answer in the following thread: http://www.oxwall.org/forum/topic/27875?page=1#post-125909
Most probably it will help you to understand the way the system works.