S.,
I totally understand your concerns. My husband and I each had a small child when we started dating. We lived together for over a year before we finally got married - just to make sure our combined family worked well together. It also gave us a glimpse of our different parenting techniques.
Since our kids were so little, we just made the changes very slowly. By the time they actually moved in with us, they were already there all but bedtime and everyone seemed comfortable. Small steps seem easier for the kids. I think the biggest challenge was my ex - hopefully there isn't an ex-spouse who is going to feel their "former" family is being invaded. Your younger child at some point may even develop more of an attachment and refer to your new guy as "Dad". There are pros and cons to this that you might want to talk out ahead of time.
It all comes down to making sure the kids don't feel that they are being replaced as your focus - our kids had momentary feelings of this when we got a cat, so we knew that moving in together was going to be a big deal. All in all, as long as they feel secure I'm sure it will be great.
Best of luck!