You cannot modify a title template. Within the template you can change text font and size; rectify the text to left, middle, or right; make the text italic, bold, or outlined; and change the color of the text. With some titles and fonts you cannot change from upper case to lower case. The Lower Third title contains only upper case. The Reveal title contains both upper and lower case with the Avenir Bold font. Here's a link that explains how to modify titles:
https://help.apple.com/imovie/mac/10.1/#/mov9c5e9641f
Titles and the Picture in Picture feature cannot be stacked to play at the same time, but more than one can be added to a clip side by side and they will play sequentially. If you want them to be stacked so that the titles or picture in picture play at the same time you can place the first one, then share out the clip, then import the clip back into iMovie, and then add another title or PiP on top. That way they will play at the same time instead of sequentially.
-- Rich