My 2x12 has a 16ohm GT12-75 on one side and a 15ohm 100 Celestion something on the other.
Couple of questions:
Is it safe to run them mono from a 16ohm output.
Do I need to make any considerations when doing so?
Can I break anything?
15ohm? If you wire them in parallel, it'll run at 8ohms in mono. Everything should be fine. I have an Eminence Legend 120w 8ohm and a Swamp Thang 150w 8ohm running in series at 16ohm mono and it's fine. As long as your amp wattage isn't over the speaker wattage (even so, that can be fine so long as you don't crank your amp full).