So if this is the case is it acceptable to have differences training from pilots who are not FIs? EG tailwheel.
There are a few specific cases where the ANO (and similar rules) require differences training
from a suitably qualified instructor. Tailwheel is one of them. So for tailwheel, the answer is no.
However, there is no such requirement for, for instance, aerobatics. So legally speaking anybody can teach you aerobatics, even if that person doesn't hold a (legal/current) PPL.
(Whether that's a wise idea is another matter altogether, and it looks like this is one of the areas that's going to change under EASA. But that's a different discussion.)