Can a Large Enterprise / Big SME (a private, for-profit company) is eligible to participate as a Project Partner (PP) throughout the whole project duration to deliver technical tasks? If so, are there any specific conditions, limits on co-financing rates, or restrictions under the State Aid / de minimis frameworks?

Based on the Interreg Euro-MED Programme Manual (p.19), small and large enterprises can be partners (but not lead partners) in a Euro-MED project:
” Partners can be:
· Public bodies or bodies governed by public law (according to the definition of the Directive 2014/24/EU).
· Private organisations
· International organisations acting under national or international law ( under the condition that the Programme’s control systems and audits are respected).
BUT
“Private companies whose main activity, including their role in the project, is solely limited to supporting actions (project coordination, management, communication, etc) cannot be involved as project partners. Indeed, their participation could fall within the scope of public procurement, and usually do not contribute to the overall project activities.
Compliance with this requirement will be checked during the quality assessment of each project proposal and through the partnership relevance and transnationality analysis.”
Therefore, the role of the entreprise in the project has to be taken into consideration to assess its eligibility.
As far as state aid is concerned, this will depend on the activities carried out during the project. A self-assessment is requested in the Application form, for each partner. This will be further assessed by the Programme bodies after submission, and reviewed consequently if needed. I recommend you to read our related guidance on p.151 of our Programme Manual.