Hi E.,
Me too! My son was weird with the food too. I was exclusively breastfeeding, he was showing food interest at 5 mo, so we tried. He did not like the textures and really wasn't interested, so I stopped. He is now 8 mo and 4 weeks and it wasn't until last week that he really showed an active interest in the food.
He is on and off, sometimes eating up to a cup, of oat cereal with veggies mixed in, to only eating 5 to 6 bites. I offer water throughout the day in a sippy cup, he loves water. But I don't think he drinks a total of more than 2-3 oz.
But when we started he really didn't get the whole eating thing with a spoon, we had to practice (that's how I thought of it).
A great book is Super Baby Food Diet. She has tons, and tons of info about what to eat, how to introduce food, storage and anything else you possibly need.
I don't think you have anything to worry about when it comes to grains for your daughter. I think she is pooping more because she is eating more. Pooping is a sign of health. If she is straining to poop, that is something to worry about (but they sure get stinky now!).
Also according to everything I have read, you are introducing foods at the right pace, you are not going to fast. By the way, I found he liked oatmeal better that rice.
Oh, also, sometimes he liked something and I'd try it again and he would reject it. Some advice I was given is that you have to intro something 7 times before it becomes familiar (even with bigger kids), and I find that works. By the third time he is into it.
To answer the Tolerance question, I don't think it's a matter of tolerance to food. Her digestive system is now ready to accept certain foods because it has the enzymes to break it down. If you feel she is having a hard time with it, then pull back to once a day and take time to build up to multiple meals throughout the day. Trust your feelings. Find the info and then make your decision.
You can visit the Dr. Sears website and babycenter has good info.