All sources I have seen do say that Slovakia was conquered by Boleslaw, just like Moravia. I think the main point of debate is when this happened, some claim that the year of conquest of Slovakia is 1015, rather then 1003. So the time might be a bit unclear, but thats pretty common with medieval history, the conquest itself most likely did happen, whether it was 1003 or 1015.
Anyway the Polish conquests were very tenuous even during the reign of Boleslaw, since he had strong opponents in the Holy Roman Emperor Henry II, the Bohemian Premyslids and the Hungarian Arpads. Particularly the emperor gave him a lot of trouble, as he wanted to add Poland into HRE and didnt like Boleslaw´s takeover of Moravia, as it was previously held by vassal of HRE - the Duke of Bohemia. And so soon after Boleslaw´s death, his conquests were quickly lost.
So I would suggest you give Moravia and Slovakia to Poland, but also give strong claims on Moravia and Slovakia to the rulers of Bohemia and Hungary, also you could give the duke of Bohemia a favour with Henry II in order for the duke of Bohemia to use this favour to regain Moravia, since historically Henry did help Bohemia by fighting the Poles. Also Boleslaw´s vassals in Moravia and Slovakia should have bad opinion of him and should be of Bohemian and Hungarian culture respectively and Henry II should have good relation with Bohemian duke and possibly with Hungarian king and very bad relations with Boleslaw. This would imo simulate quite well the historical situation in central Europe in 1018.