In 'most' router/DSL modems its necessary to list them as two separate rules.
However, that said if your uploads and downloads are working then its probably ok to leave as is. If you have problems with your browses, pms, whois..then that could be a sign of the two conflicting and in that case you may wish to make them two separate rules for WinMX.
Port forward has to be commended for the great work they do trying to keep up on all applications, and all new routers and modems.. its not easy I can assure you. So they show the standard 'Default' ports that WinMX is set to use being 6699 and 6257. There is NO reason what so ever that you cant make those ports what ever you like really. So 16699 and 16257 are just fine.. they just cant show you every port possible and the instructions are meant to be examples of how to proceed..
Let us know if you have any further troubles ..