You absolutely did not need to use Fast Pitch. It won't hurt anything, but, with dry yeast like US-05, it is completely unnecessary. Dry yeast is extremely robust and reliable. One of the biggest advantages of dry yeast is that you do not need to make propagation starters with dry yeast. A 1/2 pack of any dry yeast is enough for 5 gallons of ale, or 1 full pack is enough for lager. You have like 10 times as much yeast as you really need. You can use a small portion of it, or all of it. I don't think it will make any difference either way.
Nothing should explode if you don't have a hard seal on your fermentations. Just keep an airlock on or loose lids and all should work out fine.