Hey there! I wouldn't recommend using a smaller dildo than the diameter of the O-ring. The dildo will be floppy in the harness and make penetration/thrusting extremely difficult. You'll want a medium to hard density dildo so that you can penetrate with ease and thrust in a fluid motion.

This may sound silly but if you can fashion a sort of "glory hole" to practice with that will really help you learn how to move your hips in a fluid motion. (I assume you are a biological female and if that's true, women aren't really designed to do the thrusting, so it takes some practice to get it just right.)

If you can't do that then just take it really slowly. Unfortunately, the harness really makes all the difference here and can be difficult to find the perfect fit. I'm partial to the RodeoH line of harnesses as they fit snugly. The only downside to those is they completely cover the wearer's genital area and you can't really receive any pleasure or play there simultaneously without introducing other toys like pocket vibes and things like that.

UNLESS you go with the Feeldoe or other similar product where you insert the dildo vaginally -- this requires excellent kegel control and will be something most people need to work up to.