Hi vito,
I normally suggest that you follow the recommendation of the manufacturer but if you are in doubt a foundation with slats that are in the range of 2" to 3" apart should be fine for the futon and would be a “safe” choice. Smaller gaps are better. I wouldn’t use slats that are further apart both because the futon may sag into the gaps between the slats over time and because with a thinner mattress you may feel the slats if they are too far apart.
It would probably be fine for most people but if you are allergic to dust mites then it may be an idea to use breathable allergy encasements around both the foundation and the mattress. There is more about dust mites in post #2 here and there is more about allergy encasements in post #2 here.
Phoenix