Amy, once you get the machine back to correct tensions.....LEAVE IT ALONE! A new person with a stitcher causes 99% of their problems by fiddling with the tensions.
Before you do anything with tensions, check the thread track from the spool, all the way to the needle. It is very common for the thread (top thread) to hang up on a screw, or be misplaced through a loop, one of many other things that in interfere with the smooth movement of the tread along its intended path. This causes very tight random tension and will result in pulling the bobbin thread up to lay on top. this is a fairly common occurrence that I experience frequently because I think I'm so good that I can rush through the threading process. Not true, take it slow and be sure everything is moving smoothly. It only takes a second to double check. (I should take my own advice more often.)
I use 207 top and bottom which Im think is just about right for the sheaths I produce. I do not notice any advantage of having different weight threads top and bottom, but there is nothing wrong with that if you prefer it.
Finally, your trouble with back stitches could be your left hand. Let the machine do the work and don't force any movement with your hand. Your hand is there to guide the material only....not advance it. Also I have noticed (and I don't know why) that if you fail to use the reverse lever for back stitch and just raise the presser foot and re-positon the piece for a back stitch, the result will be about 50/50 failure for a good looking stitch.
Good Luck!
Paul