I sympathize with the heartfelt cry, "Does the English language not specify which tense that we must use when sentences are constructed like this?" The answer is sort of. "The English language" is really the speakers of the English language, and they don't always agree with each other, and they're not always consistent, especially in informal communication like the spoken dialog that you're reporting.
The first case you cite is, I think, the result of reported speech (or in this case, thinking.) If Mr. Bruce were reporting his thoughts directly, he would have told the narrator: I thought, "I will tell you when I see you." But Mr. Bruce tells the narrator a report of this thinking: I though I'd [I would] tell you when I saw you." This is called "backshifting the tense" for a past report ("thought"), and it takes "will" to "would," and "see" to "saw."
The second case has almost the same structure, but with a slight syntactic ambiguity about the reported thought. Is it of
"I will tell you."
or is it of
"I will tell you before you get to the office."
In the latter case, "get" should be backshifted to got. In the former, the temporal clause is not part of the report, and the verb "get" is in the present, which is used for near-future events.
Of course, that future sense clashes with the past "I thought," but getting the tenses technically correct would require something like, "Before you get to work, I will have thought that I had told you," and no one would say that.
There also is some sense of obligation in the sentence, not only that the speaker would warn the narrator but that he should. In which case, the sentence "I thought I should tell you before you get to work" is fine with the present tense indicating an ongoing situation.
Remember that it's dialog, and people don't always speak "correctly." Perhaps the author deliberately tried to mimic everyday speech, or perhaps he got it "wrong" in the first place.
Those are all correct, and not just in YA. Thankfully grammar has a lot of room for nuance when it comes to tense. As paradoxical as it seems, words like "this" are often preferable to words like "that" in past tense. If I read "But that trip was way different" during a portion of the narrative in which the narrator was on a trip, I would start wondering if the narrator was referring to the trip he/she was currently on or some other previous trip. The "now" or "this" or "here" refers to the moment in the past currently being recounted in the story.
For example, take this hypothetical passage: "Two years after the breakup, I looked Anna up on Facebook. She had lived in Portland, but she lived in Seattle now." Anna's living in Portland and Anna's living in Seattle are both past events, so they're in the past tense. Maybe by the narrator's present Anna lives in San Francisco. But from the point of view of the moment currently being recounted in the narrative -- in this case, two years after the breakup -- only Anna's living in Portland is the past. Anna's living in Seattle is the present of that moment, so it's okay to use words like "now."
The only thing you wouldn't want to say is something like "She had lived in Seattle now." That would be contradictory and confusing because the "had lived" implies that she lived in Seattle before the moment being recounted, while the now implies that she lived in Seattle during the moment being recounted. But "She lived in Seattle now" is fine.
By the way, to back up my statement about this being extremely widespread accepted usage far beyond YA, here's a passage from Chinua Achebe's "Things Fall Apart," high literature by most anyone's definition: "As the years of exile passed one by one it seemed to him that his chi might now be making amends for the past disaster. ... He sent for the five sons and they came and sat in his obi. The youngest of them was four years old." (p.172) The youngest son is four in the moment currently being recounted in the narrative, just as his chi seems to now be making amends for the past disaster in the moment currently being recounted in the narrative.
Best Answer
Both have their place, and mean different things.
If the narrating perspective considers after the story (whether from an indistinct "now" of the storytelling, or from a particular framing time frame) that they had surprised "him", then this would be more appropriate.
If the narrating character thought then that they had surprised, then "I thought I surprised him" might be more appropriate. (The mixture of simple past and past perfect, "I thought I had surprised him" is different again, and also has its place).
It's generally just called "mixing tenses" or "switching tenses".
It's important to note that while more basic instruction (whether to younger students, or to students of English as a second language) is not to mix tenses, this is in the category of "lies to children": Not only can one mix tenses in a great variety of ways, but one should mix tenses. The difference is between slipping from one tense into another (as perhaps might happen with a novice speaker more familiar with the present tense forms) and switching because it is appropriate to do so.
In your example of "I think I surprised him" is appropriate if present tense is appropriate for the thinking and past tense appropriate for the surprising, and not otherwise.
Really the rule is "always use a tense appropriate for what you want to express". Mixing tenses when one shouldn't is an easy way to break this rule for the most novice of English writers and speakers, but not mixing them when one should is another form of the same problem.