Hi, my baby is one week old and during tummy time in the last two days has been able to roll from his stomach to his back. Being so young, his startle is still very strong and he seems to need swaddling to sleep for any longer than 20 minutes or so. He has shown no signs of being able to roll from his back onto his side or stomach. Are we still ok to swaddle him for sleep?

Sometimes baby can accidentally (or intentionally) roll from tummy to back at an early age.
Some will roll from tummy to back (when doing tummy time) from a very early age, like your son.

You could try him with his arms out of the swaddle, if you think he might start rolling during sleep.
What type of swaddle are you using?
Certainly, if he were to roll from back to tummy, especially when sleeping it would be important to following the guidelines for when a baby is rolling:
Cease wrapping
Move from bassinet to cot.
Always place baby on back for sleep.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Some babies do roll early when sleeping, so it is important to following the recommendations.
Some will resist wrapping - these babies often like their arms free to touch their face or suck fingers.
Red Nose has the following information now that your baby is starting to roll.

If you are using a bassinette, it is time to transfer baby into a cot as soon as they first show signs of being able to roll.
Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over