Is there a time when you can't do this? I mean, if I have weak hives, can they be combined this late in the year?
Yes you can combine them, just make sure you combine them by using one of the proper methods (example:using newspaper) and that there is only one queen. Then make sure they are healthy, check food stores, i would also use the front entrance reducer and then everything else you need to do before winter. Good Luck!!
I would do it ASAP, before the bees move into cluster formation. I have a queenless hive from a late September swarm (silly Russians) that I'm going to combine with one of my nucs during the coming week.
Winter's closing in here in Maine. What were those bees thinking?