will these fans be useless?

Some fan slots come with multiple holes for different fan sizes so you might get lucky. Nevertheless 120mm/140mm fans as appropriate are better than 80mm fans due to the lower noise/airlfow ratio they produce. So considering you get two fans fitted as standard in the Antec 100 you'll need probably 1 or 2 for the front mounting and you may want to replace the Tri-cools you get with Antecs as they are not the best, they aren't awful mind, just average as a case fan.
 
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