Can I re-site a pieris now from a large pot into my small garden.

I would say yes!

Containerised plants can generally be transplanted at any time.

However I would suggest you prepare the planting hole in advance of the move.

The detail on this link from the planting stage will give you some idea as to what to do!

http://www.thegardenersalmanac.co.uk/Data/Shrubs-Transplanting/Shrub%20Transplanting.htm

I agree with Teegee..and as they like acid soil, ,it might be best to dig some into your planting hole,if you don't already have those conditions...it will give it a good start...
I don't have acid soil,but both mine are thriving,with adding this first,and I keep giving them a mulch every year..no problems at all.. both were in tubs at one point ..good luck ..

You can do it now, but you will need to keep it well watered until the weather turns much colder and winter is upon us - it won't be able to seek out its own water for some months, until it breaks out of its rootball and puts out more roots. Alternatively, wait till late September and do it then.
As Bloomer indicates, if your soil is not acid, then digging over the area you're going to plant it in and incorporating some ericaceous compost would be useful.