The first costruction example is missing a piece.

Is this slang?

No, no, what I meant was, can this be used in formal situations?

If I'm not mistaken, っぽい is actually informal. It's at the bottom of the 'likeness suffixes' (sorry, couldn't think of a better word haha).

In order of politeness/formality, I'd say it goes ~様、~みたい、~ぽい. That's not to say it's rude or anything, just informal.

When going over this point with my tutor, she said that ~みたい and っぽい really can't be used interchangeably. There's a slight contextual difference between the two. 例） あの男は女っぽいだろうね = That guy is kind of [i]womanly[/i], right あの男は女みたいだろう = That guy [i]looks like[/i] a woman, right When using っぽい its more of describing someone/something actions without a definitive... If that's how to explain it. ~みたい is more of stating a definitive, like that guy totally looks like a chick. Basically, if the guy didn't look like a girl but acted like a girl, you wouldn't use ~みたい. So, essentially, っぽい couldn't be used to write/say something like, スープはおいしっぽい. Because the sentence would awkwardly translate into something as, That soup is pretty deliciously (looking). It'd have to be スープは美味しそう, the soup looks delicious. Nor could you use スープはおいしいみたい to say that the soup looks delicious because that would then translate into something like, the soup looks like it's delicious (but on the other hand maybe not). I hope I understood that correctly from my tutor. She doesn't speak any English so it may have been lost in translation.

Well, I talked with one of my coworkers about that and she said that you couldn't simply add っぽい to any noun. Like, for instance, I brought up パソコンぽい as an example and she said that sounded weird, like no one uses it that freely. It's mostly used in situations to describe someone as something, ie あの人は演歌が好きだから日本人ぽいですよ　(That person likes enka so they're kind of Japanesey)。 or, something like this (actual role play with my coworkers, lol), 平井堅は外国人みたいな。。。　(Ken Hirai looks like foreigner) え？そうかも知れないけど日本人です。 (Eh? Perhaps, but he's Japanese.) 平井堅は外国人ぽいやな。。。(Ken Hirai seems pretty foreign (He acts foreign)) え？見えへんわ (Eh? I don't see it) But if I used, 平井堅は車っぽいです (Ken Hirai is like a car) Then that's not normal usage. You could but its weird. Maybe perhaps if it was like 自転車は車っぽいです (The bike is like a car), but even then.....