There is nothing wrong with having two young players like McGary and Sabonis battle for minutes. Even if they were similar (they are not), Sabonis is a good insurance policy against McGary washing out completely (something you hint at). Also, the only similarities between the two players are their height, their skin colour, and the position they play.
If you’re looking for a comparator for Sabonis, it is Kanter. He’s probably Kanter-lite offensively but has the potential to be much better on defence due to his toughness. Both are good rebounders. Maybe Sabonis turns into Kanter with a 3-pt jump shot and better defence— that’s a very good haul for the 11th pick. As Golden State has demonstrated, you can never have too many players that can do a little bit of everything.
To your second point, defensively Oladipo is a great fit beside anyone. He has decent size coupled, good hands, and phenomenal athleticism. He’s active and has a good motor. Size is not an issue. There are maybe 3–4 guards in the league that can hurt him with their size — two are in the Eastern Conference (Butler/DeRozan), and one is a point guard (Livingston). Include SFs and the list grows. However, that goes for most SGs not named Butler/Thompson. The Thunder have been looking for a legitimate 2-way shooting guard for a long time and Oladipo fits the bill nicely.
Offensively, he is a so-so fit with Westbrook. They are going to be awesome on the break given their speed, athleticism, and ability to generate turnovers. In the halfcourt, they are going to be rather mediocre unless Oladipo develops a reliable 3-pt shot to open up driving lanes for Westbrook.