I've been there-- my son had TERRIBLE reflux and we were told by his pediatricians (who are all also lactation consultants) to add the cereal to his breastmilk/formula at about 3.5 months-- and we started him on solids shortly after that as well, which is way earlier than the conventional wisdom is now, but you have to make decisions based on the child's needs. And, of course, we had the constipation problems then too. I would go back to adding 1/4 teaspoon for every 4 ounces for 3 or 4 days, then 1/4 teaspoon for every 2 ounces for 4-5 days, and then up to the 1/4 teaspoon for every ounce. Some other things to try to help with the constipation:
Warm baths (this can get messy, fast)
Infant massage-- google "infant massage constipation" to get some ideas
Putting him in his bumbo chair (that usually worked for us)
Highly diluted (like, start with 1/2 ounce in a whole bottle of water) pear juice
glycerine suppositories (Last resort, and check with your ped first)
Good luck-- this WILL pass, but my son was still spitting up at 12 months, so it can take a long time!
It is a tradeoff, but if the reflux is that bad, you do what you got to do. :-)