I can't be bothered reading through every comment to see if anyone already said all of this, but unless you're downtown in a major city, throttling shouldn't ever really be a problem. My parents just switched to the new unlimited plan from the old one less than two months ago, so I got an iPad with unlimited data and last month I used 22GB halfway through the monthly cycle. I was never throttled after that AFAIK. Remember, you aren't throttled after 22GB. There's just the potential for you to be throttled sometimes when the network is congested.
Also, you said you were going to get the 15GB plan, which costs $100 plus $15 for each phone. If you were switching to this plan yourself, the unlimited plan is $15/month cheaper. If you add any more lines, the 15GB plan becomes cheaper, but all you're doing is stretching that 15GB among more people. Every line you add to unlimited gets at least 22GB before ever being throttled, so two people on unlimited would be like having 44GB to share, three people on unlimited would be like having 66GB to share, etc. except even better because you don't get charged more after you use that much and chances are you won't be throttled anyway.
Also, you said you were going to get the 15GB plan, which costs $100 plus $15 for each phone. If you were switching to this plan yourself, the unlimited plan is $15/month cheaper. If you add any more lines, the 15GB plan becomes cheaper, but all you're doing is stretching that 15GB among more people. Every line you add to unlimited gets at least 22GB before ever being throttled, so two people on unlimited would be like having 44GB to share, three people on unlimited would be like having 66GB to share, etc. except even better because you don't get charged more after you use that much and chances are you won't be throttled anyway.
