When I use a substitute cup like yogurt containers, I always put a spoonful or so of straight Perlite in the bottom of the cup to help the holes not to plug up with the starter mix. Soggy soil will kill off your seedlings in a heartbeat, and that's very sad.
Also, I agree with woofie, put the squash and zucchini seeds in the larger containers. They get big fast!
You can (probably should) put out your Romaine lettuce and cabbage much sooner than April. They like cool weather, and lettuces bolt to seed if it gets too hot too fast. They can also develop a nasty bitter taste if the temperatures bounce from cool to hot a few times. Go for late March for the lettuce and cabbage, to take advantage of all the cool spring weather. Cabbage-y things can even take a light frost.
