Most after market alloys require centralising rings... the bore for the Yaris is 54.1, rings for OZ wheels to Yaris shouldn't be hard to get hold of.. couple of quid each.
QUOTE
Centralizing Rings
These are a very important, and often overlooked part of buying aftermarket alloy wheels. Centralizing Rings are the plastic/metal rings, that should get supplied with your wheels, that will convert the Bore Hole of the wheel (the hole in the centre) to the same size as the hub.
If the Centralizing Rings are not fitted you have 2 problems..
1.) The wheel won't be centred on the hub properly, resulting in vibration through the steering wheel, this feels excactly the same as driving with wheels that are out of balance.
2.) This is a problem that most people won't even think of, if the Centralizing Rings are missing, the wheel will have no support from the hub, this means that the full weight of your car is only being supported by the wheel nuts, which they are NOT designed to do.
More usefull info on wheels etc, can be found here.....
http://toyotaownersclub.com/forums/index.php?showtopic=40364