that's correct, it only has a single power connection so won't do multiple cards. but it also won't run a card that needs more than 1 power connection. no idea yet if custom cards will need it but it is common for custom cards to use more power than reference ones.
for instance the new 1070 and 1080 use an 8 pin connector. some of the custom cards use 2 x 8pins!! many use a 6 and an 8 pin. so just have to look first to ensure you get one that only needs a single connections. 6 or 8 pin is fine as you have that covered.