So consider Pepe a SS and Quaq a CF and what a forward like Treze, Vieri, Etoo, Toni and Klose are then?
You're being way too limited in understanding their roles on the field. What you're talking about is the old fashioned system put on paper. You must have noticed by now how the game has changed.
Anyway, Pepe was never called a SS, lets not put words into peoples mouths. He was called an attacking player and part of the front three in Conte's 4-3-3 system, which isn't a 4-5-1 as you call it since both Pepe and especially Vucinic are almost never in the same line with the central midfielders. Pepe is a forward in Conte's system simply because he plays there. And yes, he's supporting the central forward up front so technically you can call him a SS, but in the actual understanding of the SS role, he definitely isn't one.
As for Quag and the other forwards you mentioned....yes, he's playing the same role as them, just that he wasn't blessed with the goalscoring instinct as they are. He always goes for the goal and rarely, almost never, for the pass. He can't do much playmaking either and he definitely can't function from the wing and cut inside. His instinc will push him to the center of the box and we will lose width since he won't go back to the wing when needed. He has more characteristics of a CF than SS. Naturally, he can play in a forward duo with a less mobile and taller striker, but again he will play as a typical CF there.
To make it shorter: Yes, in our formation Quagliarella is a direct replacement for Matri and he'll do the best if he plays instead of Matri while he'll struggle if he plays instead of Vucinic and Pepe. He can play together with Matri of course, but to get the best out of him there, you'll need to take out one of the CMs, take Vucinic out and change the formation, by adding a winger.
To make it clear for you, these are the 10 goals he scored for Juve. Maybe they'll tell you more.
01. Sampdoria 4-4-2 (Quag-Del Piero)
02. Udinese 4-4-2 (Quag-Del Piero)
03. Lecce 4-4-2 (Quag-Amauri)
04. Milan 4-4-2 (Quag-Del Piero)
05. Cesena 4-4-2 (Quag-Del Piero)
06. Brescia 4-4-2 (Quag-Del Piero)
07. Catania 4-4-2 (Quag-Iaquinta)
08. Catania 4-4-2 (Quag-Iaquinta)
09. Chievo 4-4-2 (Quag-Iaquinta)
10. Novara 4-3-3 (Esti-Quag-Pepe)
Oh, and in 30 matches for Juventus he made 2 assists. Not really good for what you consider to be a support striker, isn't it? One of the assists wasn't even an assist because he was shooting on goal and accidentally hit Pepe. That was last year's match against Sampdoria.