You're right, pagination is not implemented for the Condition resource in Cerner's sandbox. As you expect, all Conditions except those filtered by the request parameters we expose will return.
I would however recommend that you implement your Condition queries in such a way that paging is handled as we may add paging in the future to resources that do not currently page. If you are using an existing reference library like the .NET API you should be good on this front.
You can search the returned conditions list for diabetes via Condition.code - for example in the sandbox environment we have http://snomed.info/sct|44054006
mapped for Type 2 Diabetes. Depending on which representations of diabetes are important to your workflow, you can filter the conditions list accordingly.