No, you'll be fine.
They state that so that people who only have one costume (the base slot) know that they need to open up more costume slots before they can take advantage of the costume change emotes.
Let me never fall into the vulgar mistake of dreaming that I am persecuted whenever I am contradicted.
~Ralph Waldo Emerson
"I was just the one with the most unsolicited sombrero." - Traegus
No, you should be good to use the emotes. Go into the costume screen and there is a drop down box at the bottom for the costume change emote. Select your emote and then click on the costume you want to change to.
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Unofficial leader of PAAS (Paragon Association for the Advancement of Stoners)
Or, if you want to make a keybind:
/bind <key> cce <costume slot #> <emote>
example:
/bind numpad1 cce 2 CCBackflip
List of emotes is here.
And an important thing to note is that the costume slots start numbering at 0 rather than 1. 
cool...thanks.
I would like to buy the booster pacs for the costume emotes. it says in order to activate the costume emotes you have to unlock another costume slot.
what if i have unlocked all 4 of my costume slots already? am i SOL?
thanks.