If you choose to show your pony in the foundation halter classes and there are no foundation performance classes or--as I have seen at some shows--no youth or amateur foundation classes, then you can show those classic classes. The catch is to look the show list over VERY carefully and see what is offered in foundation and what is not. You cannot--however--show your pony in foundation and then in Modern or modern pleasure. It must be classic--
If you are showing he/she foundation, there are some classes that are not offered just for "foundations" and they can cross enter into such as Classic Liberty, Classic Jumping ext.