Is it ok to use fake words like that?
If you are making up the culture, that is fine. Slang changes, year by year and city by city, all over the world. Some sticks longer than others, some even become part of the language (like "cool"). Others fade out ("the bees knees" used to mean very cool).
Part of what makes them stick is brevity in speech, single syllable words that are quick and easy to say (few mouth or tongue movements) slang sticks better than multi-syllabic slang. "Hot" for "sexually attractive" is likely to stick, "Cool" for "interesting, awesome, fascinating" is too. But that isn't a concrete rule by any means. "Fuhgeddaboudit" ["forget about it"] is still popular, "Tots" ["totally"] may be on the wane.
"Funstuffs" doesn't sound like slang. Say this in slow motion and notice how much work your mouth and tongue must do, compared to the simpler work of "cool" or "hot". It also saves no time from "fun stuff", is not easier to say, and is not an imaginative or interesting take on what it is supposed to represent. One purpose of slang is to form a "private language" for a group, that makes them different than outsiders (and helps identify outsiders).
Made up T-Shirts, made up spellings, advertisements are fine.
It can do two things for you: First, it can be entertaining to readers. Second, it can immunize your work against changes in slang, which are inevitable. Every group of teens everywhere, as part of their biological urge to gain independence from their parents (so they can start love lives and mating without repercussions), will form a culture with language they know and adults do not: The modern version is abbreviated acronymics in texts that are easier to say (or type) and become words themselves (eg LOL).
This means using REAL slang is likely to "date" your book (or work), it can seem "old fashioned" and boring. Original slang that you invent won't seem that way, to each reader it will seem like new slang, which is better than out of date slang.
If your slang is not instantly understandable (and it should not necessarily be to seem like slang, remember adults are supposed to be confused or unaware of what is being said), have your adult of the time "translate" it by restatement.
"Wow, what's with all the deadgear?"
"It's old but it works. Come on in."