Re: slow vs fast twitch muscles -
I've never heard any athlete say "I wish I hadn't lifted so many weights early in my career" or "I wish I wasn't as fast as I am". Worry about the needs of advanced rowing when the time comes for that, if I were 16 I would be trying to become the best athlete possible, which means spending time in the gym lifting and getting strong.
I've never rowed competitively, but I 2k in 6:45, which isn't glacial, and I saw a very direct correlation between my rowing time and the weight I could clean. This correlation has slowed down as the clean as gotten higher, but I feel comfortable in thinking that for a rower, priority #1 is getting the strength up to an acceptable baseline (clean 200, squat 300, which might be high for a 16 y/o but are still good goals to aim for eventually) while rowing regularly, then worrying about specializing from there.
I have a buddy who rowed competitively and 2k'ed somewhere in the 6:15s or 6:20s, I will ask him if he has an opinion on the issue.