Hi,
a) If a large number of tours (over 100) are linked from as many websites and are calling the same set of xml files and pngs images will this slow down the tours
no, the opposite will be happen, because these files can be cached better in this case - and - I'm not sure, but I think for the server it will be also better to serve always the same files instead of many different ones (e.g. because of internal optimizations like file-in-memory caching)
best regards,
Klaus