Placing the clip you want to change on a completely blank video timeline shouldn't change the other video timelines, so try that first. If that still moves your other timelines, then the the best way around it is to first confirm the amount of time your new clip needs to run. Then, right-click, select "Time Stretch...", and then set the clip time in the window that pops up to your new clip time. The Time Stretch right-click function doesn't move the rest of the timelime when you shrink or grow the clips.
As to Titles, when you create a title, it will always use the next available unused video track at the point on your timeline the slider rests. So, if you click the Add Title button and your slider is at a point with 3 full video lines, the Title will add to the 4th. There is no way to change that, as far as I know.