R.N.
Alexandra,
Some of the new literature suggests to stray away from soy because of its estrogen like qualities, especially for baby boys, unless the benefits outweigh the negatives. Unless your family eats a diet where cow's milk is not eaten, I would definitely use it (milk-based) first. Usually soy is used second, only if regular formulas don't "work", and even then, you can get goat milk formulas now too. See http://askdrsears.com/html/3/t032400.asp for info on goat's milk in general.
FYi-Good start is the best commercial formula as far as gentleness goes. My son is allergic/intolerant to casein, one of the components in milk protein. So, he could only drink Good Start because it has only whey proteins, or as they call them "comfort proteins". These are easier for kids to digest (per our GI dr) and it's the best formulation to use if your child is sensitive (gassy, etc.).
As far as the constipation...my son had that starting at about 5 wks. We tried the laxatives and rectal stimulation. Then, when I switched peds, the new one said to reserve those for times when the child has not gone for 7+ days and it is extremely severe. Another sign it's severe is blood from a tear and /or pebble-like stools. My new ped recommended 1 oz of prune juice (even apple juice works some times) each day. It really helped. My son did go 8 days once w/o pooping and it seemed so painful when he finally passed that one!!!
Good luck!
R.