I am not an expert, but I did grow about 24 trays of plant from seed last year with mostly great results. I tried Lobelia, Echinacea, Galardia and Petunias from your list.

The Lobelia can certainly be in a shallower pot, their roots are not deep.

Echinachea will probably not bloom the first year, but should come back gangbusters the second, so a smaller pot, even if it stunts them a little should be fine.

Petunias have pretty extensive roots, so I would pot those up in the larger ones. I had mine in 9oz plastic cups last year (twice as tall as wide) and they did well. (as the petunias send out trailers, you can snip them and root in pearlite. I tripled my number of plants last year)

Galardia (I grew dazzler mix) also did well in the 9 oz cups. They bloomed profusely by mid July.

There arent many things that will do well in 2 1/2 inch pots for as long as you will need to keep them. Do check out the plastic cups at a dollar store, or Wallyworld. You can get a lot more height for the roots with out getting a lot wider (which is what takes up the light space).

Thanks for that info. I did mis-speak. Planting out usually isn't 100% safe until mid May which is more like 10 weeks instead of 6. (wishful thinking I guess). I am determined to get a bloom on the echinaceas; but I realize that determination is probably futile. :) I'm trying to use square pots so I'm ordering decent ones so that I can use them again. A little more expensive than plastic cups from the dollar store; but then I get a bit more soil for roots to expand into. Since it is such a small operation; the cost isn't too great.