Can I use the 10G slots on the DGS-3426, DGS-3426P, DGS-3450 and the DGS-3437 for both stacking and uplink when stacking mode is enabled?

1. For DGS-3426, DGS-3426P and DGS-3650 with stacking mode on:

No you cannot. When stacking mode is enabled, both the 10G slots can ONLY be used for stacking.
For example: You cannot use one 10G slot to stack and use the second 10G slot as a regular 10G port for uplink to a server, backbone or other regular 10G port.

2. For DGS-3427 with stacking mode on:

10G slots, port 26-27 can be used for stacking only.
10G slot port 25 cannot be used for stacking. It is a regular 10G port that can be used to uplink the switch or stack to a server, backbone or any other switch with 10G port.