DISCUSSIONS on “Rules for online games in HoMM3 and formulas for calculating player rating.”

The rules don't prohibit this. It's also not forbidden to buy and upgrade monsters from hidden locations, Pandora, etc.

And another question: is capturing an opponent's empty Castle considered an attack? For example, if I'm playing as Red and have troops from the new weekly reinforcement on the first day of the week, can I use them to capture empty Castles belonging to my opponent?