A 12 inch frontosa can easily swallow a 6 inch Altolamp.
6 inch is pretty much max size for them, though i have seen a few rare 7 inchers.
So define safe.
To me safe would be never eat the Altolamp.
If your ALpha goes 5-6 years, and THEN eats the Altolamp, would that be considered safe?
There is a good chance, that if your frontosa is well fed, they won't even bother to eat a 3 inch Altolamp.
However, many of us who have had frontosa for years know that out of the blue, a small fish gets stupid, and the frontosa even though well fed will take that bite.
Point is there are people who have had 6 inch fish get munched by a 9 inch frontosa.
My suggestion is really that nothing smaller than 50% the size of the frontosa is safe.
good chance that a 6 incher Alto would make it.
I'd not do it because i have seen them killed more than a few times.