You are correct that NPM is the only product that support stackable polling engines today. You can still stack APEs even if you have other modules, however those modules won't be able to take advantage of the increased element counts. For example you could have a single server with NPM and SAM and add a stacked APE to get the following element counts;
NPM ~20,000-24,000
SAM ~8500-10,000
Hope that helps.