TOO many chicks, different ages of chicks too-- help? Suggestions

Help I admit I am a chicken newbie but chose the Jersey Giant breed --- and now have a predicament-- have simultaneously ordered 15 hen chicks that arrive in a week-- and got a confirmation back from a local craigslisting ( I like to honor my committments) so will be getting 6 3 week old chicks as well the day after tommorrow!--

not sure how to do this-- my coop is for 8 standard chickens so I was thinking maybe 6 full grown giants would fit into it (we have an acre back yard so there is room to expand) and a second coop? How to mix the two-- and when-- I am assumming that in 3 weeks I can put the chicks outside by which time the older chicks will be 7 weeks--

also am thinking to trade off any roosters (the 6 from Craigslist are unsexed which is on reason I ordered hens online)...

Or trade some for another coop?

I was assuming there would be some chicken mortality as well so wouldnt have exactly 21 chickens... But still I will have too many...

Any thoughts, advice would be really really really appreciated-- I am in the Grass Valley, CA area!

Well we built the Purina chicken coop-- its pretty big  about 4x5 ft and at least 5ft high but I think I need another one. I want to build a run (8x8 ft) attached (we have dogs too) so I can put their feeder and waterer in there but plan to leave the door open so they can free range during the day when I am around and once we have figured out the dog- chicken situation -- Because they are levelling the septic mound I have lost most of my fencing so the acre wont be available until I get it back /septic done (2-4 weeks )...

I think I was aiming for a flock of 8-10 birds to start, but just ended up with too many ordered birds-- I think I will have to put an ad on Craigslist or... may do a trade/sale on this forum to decrease my chick numbers...

Its in the 70's now during the day, but still upper 40's at night/low 50's for a good week or so in my area....