You are correct that in much of the world uses "class ratings" for SEP - my JAA licence shows SEP as a class rating, and my FAA SEL, and whilst good practice is to get checked out by a grown up, it's seldom legally mandatory - certainly below 2000kg. This is, I think, a peculiarity of the South African licencing system.
I'd venture that you need to talk to your employer - that is the organisation you're checking people out for, and ask them for a clear statement of requirement. Are you looking for minimum acceptable levels of type knowledge, or are you looking to deliver pilots competent to do a particular job? That they need to define, and then you perhaps can cover your backside by informing them in writing of the specific limitations of what you do.
For example, I do quite a lot of syndicate checkouts here in the UK: usually pilots who are buying shares in, sometimes quite interesting, SEP types. I can pick a point on a sliding scale that looks something like:
(1) They understand the aeroplane
(2) They understand the aeroplane, and are demonstrating consistently safe flying
(3) They understand the aeroplane, and are demonstrating both safe flying in it, and would be able to pass a PPL skill test in it
(4) They are flying it as well as I - a CPL and instructor with substantial hours on type can fly it.
It sounds like you delivered (1), I'm usually shooting for (2) within syndicates, but if somebody is upgrading their licence to fly the aeroplane [for example a microlight pilot switching to SEP or vice-versa] and will need to shoot for (3) to pass a test or I need to sign for differences training. (4) is a luxury I allow myself and am happy to offer training to this standard to anybody prepared to pay for it, but strangely enough have had very few takers.
Regardless however, you, your employer, and the pilot you're training, all need to understand what you're trying to achieve on that scale. It sounds like the problem you have is that this wasn't universally agreed.
G