is it possible to make the maximum stacksize increase by level. Maybe by adding some code or by linking 2 plugins?

it'd be a hassle because it would need to be compatible with all stacksize modifying plugins. it should be implemented by those plugins instead, which would be far easier and prevent these issues.

they can easily do so using the api from zlevels, api_GetPlayerInfo(ulong playerid) or GetLevel(ulong playerid, str skillname) and scaling stacksize based on some configuration options in their plugins