Why would he more than likely be a C off the bench? He's not allowed to play PF off the bench? I don't get your logic on that. Again, at 20 mpg he'd match Dirk Nowitzki's rookie year and Manu Ginobili's rookie year minutes and both were older than him. Just to add to that, Bargnani played 25 per game and was still a bust, but Vucevic played 16 mpg as a rookie and has steadily developed into a quality player since. Their development has to do with how the teams train them and their own mentality a lot more than simple minutes on the floor.
As far as thinking about how he affects Melo, you have it backwards. How does playing in the starting line-up affect Kristaps...because Melo's the obvious first option and we're actively targeting wings and bigs, which means he's not the second option either...possibly not even the third option. What can that potentially do for his confidence if you push him into a role to defer? He's not gonna hurt Melo, Melo will get his regardless.
He has a tweener body. His biggest weaknesses come from strength problems in the post and on the block at both ends of the court. He can get away with playing PF minutes but expect to watch him get abused against the bigger, stronger front courts in the league until he packs on more weight and core strength.
He's more likely to play the 5 off the bench because the Knicks don't have the money to get a quality starting big and a quality big off the bench, which would leave Kristaps in the position to play minutes at the 5 if he's not starting. The Knicks lack depth, center is the hardest position to find in the league.
Dirk started half the games that year and was playing almost 30mpg to close out the strike shortened season. Vucevic isn't an international player like the guys we're naming, he played 3 years at USC

Again, I have more examples of foreign players getting a lot of minutes early and being successful, you named Bargnani to me naming Dirk, Parker, Ibaka and Giannis, shyt we can even take it to Yao too. I mean even the Jazz threw Exum into the fire last year and he clearly wasn't ready for it but it helps build confidence and lets them know exactly what it is they need to work on. I wouldn't give two shyts how he affects Melo, his development would be my priority considering I passed on Hezonja, Mudiay, Winslow etc.
Kristaps isn't ready to be a second or third option, but he should be playing with the starters, you have a 7'2-7'3" PF that is mobile, if you sign Greg Monroe he should be playing with him as the weakside defender just off the fact he's mobile as hell for his size and needs to learn how to defend. Also, Nerlens & Anthony Davis were both under 230lbs for their rookie seasons, you can get away with him being your starting PF as long as you have a center. There aren't that many physically imposing frontcourts in the NBA right now.