I've raised six boys so I think I'm qualified to answer your question!
My oldest is now 27. My husband, who is from Thailand, had a very helpful suggestion when he was little. He said that in Thailand, the first girl is the second mother and the first boy is the second father. Your two are still small, so you're in a good position to practice this.
We gave our oldest--and the other older boys--responsibilities from the time they were three or four. (Btw, my oldest was a bit of a terror at two and three, but he settled down by four.) It was little things like picking up toys and clothes and helping pick up unbreakable dishes at dinner time, etc. They learned that they were important members of the household.
This was really my secret. Kids who feel responsible are, in general, better behaved.
In terms of sibling rivalry, I took a page from my mother and simply didn't allow it. I stopped it at every turn. If two kids fought over a toy, I took the toy. My mother let us know early on that fighting with each other, either as children or as adults, was not acceptable. Now we're in our 30s, 40s, and 50s and closer than ever. In fact, the sister I never thought I could live with may come live with me, and I'm excited at the prospect.
With just a few simple guiding principles--I'm sure you have some of your own--you can successfully raise a large family. And it pays off wonderfully. I just got out of the hospital and my two oldest sons came to town to be with me. At night, because I have trouble walking still, they help me go to the bathroom. They help my 14, 16, and 19 year old who are at various stages--the 14 year old is starting high school and we have the other two learning to drive. My 23 year old lives about 20 miles away and comes often, whenever he doesn't have to work. Last night he and his roommate invited my husband and the boys over for dinner.
Having a large family is even better if you move often. We moved often also, not so much from necessity as from my nomadic yearnings. They were and are friends to each other. And, of course, they always pulled other boys into their circle. I have many "seventh sons."
I strongly recommend a large family. It was better for my kids to learn how to live with others, and to have that great group of siblings. And, though it was sometimes tough when they were small, I had the energy. (I was 30 when my 3rd son was born and nearly 39 when I had the last one.) Now they're here for me. They can carry me and take care of me. And they're all very close so we have a great family network.
One last thing. Money was a little tight. But there are thrift stores and Aldis and all kinds of ways to save money, especially these days. I still wait for the 5 cent sales on school supplies, though I have only two kids in school anymore. We have three in college, but one pays for himself and another one has a good scholarship. FAFSA is our friend. And the kids start working when they're 16, saving for college and helping out. It's tight, but it works.