My babies get 2 formula bottles every day, 2 breast milk bottles and then I nurse them at night (b/t 10pm and 7am). They started on formula when they were just 3 days old so they've been used to it for a while. We never had any issues with nipple confusion. We use a myriad of different bottles and nipples too...Born Free, Tommee Tippee, Dr. Brown's, Similac, etc. They take all the bottles just fine.
A level 1 nipple should work fine for you as long as you're not using a formula for acid reflux that has rice cereal added to it - if you are you'll need to use a level 2 or even level 3 nipple.