By the thumb method are you meaning the backward loop, where you just fold the yarn over itself to make a loop and then put the loop on the right hand needle? I use that a lot for a few stitches like you say. Some people say they have trouble with "droopy stitches" as you are saying (I don't, and don't know what I do differently from others). Here is an entry in Techknitter's blog about how she solves the problem with the backward loop. LINK
I'm afraid if you are having trouble doing the cable cast on, you might also have trouble with this (I'm guessing you are having some sort of space issue.)
The only other way that I know of to add the stitches would be a regular knitted cast on which is just like the cable cast on but instead of knitting between the first two stitches at the end of the needle, you knit right into the end stitch. I don't know why that would be any easier to do though.
I'd use the backward loop. Although I actually make mine the opposite to what TechKnitter shows, so maybe mine is a forward loop. :-) I don't know that that makes any difference and it works just the same.