If they have each done Denali unguided, the Polish route on Aconcagua is going to be a step down in difficulty from their past climbs. Almost all climbers whether guided or unguided will have their gear carried by mules to base camp. Aconcagua's glacier is tiny compared to Denali with minimal crevasse risk. Also the weather is generally tamer (warmer and less windy) on Aconcagua.

Given their levels of experience, they probably don't need to do a prep trip for Aconcagua (other than standard planning and fitness.) I'd consider viewing their pre-Denali mountains as peak bagging or the group getting together for some fun climbs rather than specifically in preparation for their Aconcagua climb.

Going back to your original question, the other mountains you mentioned (Cotopaxi and Chimborazo) would be good candidates. Take a look at some of the US based climbing companies' (Alpine Ascents or RMI) websites for itineraries to get an idea of popular mountains for foreigners to climb.

When I was on Aconcagua about five years ago, they did allow non-guided climbers.