Hi Roberta...
Your question is good..however...us "pros" use plug trays in many ways. FIrst, it depends on the size of the plugs...for instance, I use 501's- that means there are 501 tiny cells per tray. Those would be for things with super tiny seed...begonias, petunias ect. It is unlikely you are using those though. There are many, many sizes. My guess is you have 72's or, perhaps 35's. Does not really matter..the answer to your question about soil is what you need. Use the BEST stuff you can find, a soiless mix (all peat based). The best thing you can do is pop into a greenhouse and ask if they will sell you a bag of their mix(it is a commercial blend sold to growers, many different brands). It should cost you about 15-20 dollars. It will be a big bag, not to worry, you will use it all. They may also only have it as a compressed bale...'bout 35 bucks. The soil you use in a plug tray is the most important thing. Yes, plug trays are easier to use and save a lot of space. The top-notch soil mix will pay off in everything you plant. It is very worth your time to find this stuff. BTW..yes, there are specific mixes that can be bought for plug production but you do not need this. You need the all purpose greenhouse mix.

Your other question...I used to live in Rockford and was a grower there too. I got smart and moved to the land of sun (TEXAS!) but I remember well the schedual for perennials. I assume you plan to sell them as green plants. If you had a greenhouse it would not be to early to plant but since you are doing this in your house, the light levels will be way to low and your seedlings will probably stretch and be a sickly mess in very short order. If it were me, I would wait until March or so to get started. This way you can at least be moving your trays out into better light on warm days. A better way is to start your seedlings in the mid-summer and winter them over (once dormant, a garage will do pretty well to protect them). As spring comes around, you simply bump them up to a bigger container and you will have blooming plants to sell (at much higer prices). Growing stuff in Illinois is frustrating this time of year- even when you have a greenhouse. The quality of light is just so poor.
I hope this helps a little....your questions are actually rather big and I know I only have touched on the answers.
Hang in there-
Tim

I buy the large compressed bales of ProMix for all of my seed starting. The trays I bought have 128 cells and I have already planted all four with perennials with tiny seeds. So, if they get too leggy and ugly to sell at the farmer's markets, I will plant them here on the farm and try your summer-planting idea.

Thanks for your response!