I think you are not going the right way...I'd say this...

TModel - base
methods .Load/SaveFromStream/File(ModelFormat: TModelFormat; ...);
TModelFormat = (mf3ds, mfMsa, ...);

the specialized render model classes are then fine....I'd say that you want to achieve that you can load the model from any resource and continue working with it in the same way so no need to declare special classes for loading/saving if the run-time data representing the model is the same (that is T3dsModel or TMsaModel has no extra data)....