Whew, sorry, my computer got fried so I'm back to using my old one. Sorry to be afk.
Anyway, the correct answer is either the factories or the dropships, depending on which you're more comfortable implementing. Dropships can hold at least 4 options or at most 8 options, depending on the size of the units stored within, while factories (excluding zerg ones, which would break the 2-3 second rule) only have at most 4 each, which could become a hotkeying nightmare (actually they both might, but sadly its as close as you could possibly get.)
One more option (similar to pinecone's idea), though probably too complex for most people, is to do something like this (my creation): Since 30 is a number less than the binary number of 32, you could produce this system (though you'd have to have a quick hand with hotkeys) in no more than 5 units total. Either factories, reavers or carriers (numbered 1-6, and in seperate locations), or even dropships will do for making the selections. To get the option number you want, simply select any of the 5 options within a 2-3 second trigger-timed window you need to equal the bit flags of the number you want, and then ta-da! Despite what people think, its actually mind numbingly simple to do binary numbers. Simply treat each option as "add to" instead of "set to", and then give them the following values (for options 1 to 6):
- Option 1 adds 1
- Option 2 adds 2
- Option 3 adds 4
- Option 4 adds 8
- Option 5 adds 16
Make sense?
Anyway, interested in the next scenario? This one will be a tad more tricky.