I've never heard of expensive soil before...
A small bag of potting soil, bag of perlite and bag of vermiculite would only cost about $10-15. And you could skip on the vermiculite.
Usually has to do with how plants respond to their conditions.
Harder to grow strains would be more affected by temperatures, humidity, feeding, watering etc. So basically its easier to cause harm to those kinds of plants if they're not given ideal conditions.
-CFLs keep them around 2-4" above seedlings and you should be good (Which is about the distance from the cubes to the top of the cover)
-I wouldn't worry about the temperature too much, the seedlings would like room temp. but a little below won't cause you problems.
-If you let the jiffy pellets go dry there's a good chance you hurt the seeds, might just slow down growth, might have killed them.
-Can't tell from pic but looks like you have the CFLs setup? If so they need to be closer.
-How cold does it get in the room during the night cycle? Unless its...
They probably won't get to 6ft. Plants usually tend to double in size through flowering, some strain could maybe get closer to three times the size.
Realistically they'll probably stop closer to 4ft.