By changing orders you actually generate different features.
The a.ss01.smcp is a small cap version of a.ss01 but not a stylistic alternate of a.smcp (the latter should be a.smcp.ss01). It depends which one you want.
In the case of small cap ij acute, or just ij.smcp to make it simple, I think you are making normal IJ and the one like in Rijksmuseum, which means there is no lowercase ij.ss01 (i.e. stylistic set will be activated only within small cap). The order of suffixes should be therefore ij.smcp.ss01.