Controllerless Networks

I know it is possible but is there any concern about mixing different models of APs in the same cluster. eg 205,305,315?

There is no issue mixing it up. However IAP-205 is much older than the rest and is not supported on the latest Instant version like 8.x

So in this cases you need to go with the last version that is supported by the olderst AP  model.

The latest Instant version that IAP-205 is supported on  is 6.5.4.x where as IAP-305 and IAP-315s can use version 8.5