[Sashimister]

Originally Posted by KyleGoetz

But again, I'm really guessing here. Maybe you can say 背が高くて体が大きくなってきた。

This sentence sounds pretty straining to me. I would say it 背が高くなって体も大きくなってきた. Even a more natural way to say it would be 背が伸びて身体も大きくなってきた。

（I just prefer writing からだ as 身体 rather than 体. OP, never mind me if your book gives you 体.）

If a native speaker ever said 背が高くて体が大きくなってきた, which he wouldn't, it would probably mean "(Someone) is tall to start with and now his body has gotten bigger as well."

Originally Posted by pacerier

　regarding 「なら」 conditional, will this be a valid sentence: 遅いなら家へ帰らなければなりません

It's grammatically correct but the chances are you won't ever hear it or say it. The only situation I could think of where that might be said is the following.

You go to another town to see a friend. Have a good time together. Before you know it, it is getting dark outside. You need to catch the last train home but your watch has stopped and so has your friend's! There are no clocks in the house, either. Your friend says he will hop over to his next-door neighbors and ask. Before he steps out of the house, you might say to him 遅いなら家へ帰らなければなりません, meaning "If it in fact turns out to be pretty late, Ill have to go home." Can't believe I even wrote this. May I just say "Forget that sentence." next time?

as for 「と」 conditional, does it give the "that'll surely happen" feeling. if i were to say this これを食べると絶対許さない, does it sound "stronger" than これを食べるなら絶対許さない or is it that the listener won't really notice the difference.

これを食べると絶対許さない sounds both strange AND wrong. Does your book say this?

これを食べるなら絶対許さない is good and natural.

Best and most natural is これを食べたら絶対許さない.
_________

When you use と, it means exactly what you and KyleGoetz said. "B happens as a natural result of A." It's automatic. B occurs like a chemical reaction.
However, 許す/許さない is a matter of personal emotions/opinions, not an automatic chemical reaction.

The following are correct:
冷凍庫に水を置いておくと氷になる。 冷凍庫（れいとうこ）= freezer
この薬を飲むと眠くなる。
春になると桜が咲く。
サシミスターと結婚すると幸せになる。<-- automatic, guaranteed!