Wall design - Kreidezeit Gekkosol paint

Hello, we are really desperate and running out of time... We want to do somewhat ecological wall design in our new building (open wooden beam ceiling on the ground floor & open gable on the upper floor, so no normal ceilings)! We met with 2 painters, and both sounded totally good!!! The first would take gekkkosol fine-grain from Kreidezeit and definitely prime, but the seller of the Kreidezeit products said you don’t have to prime... The second painter would take Contact Plus from Keimfarben, said you absolutely don’t have to prime. But the information sheet does mention something about priming, I really don’t understand anymore!!! Who should you believe... Our substrates are: gypsum boards and Fermacell boards already spackled in Q2 quality (joints with mesh tape etc...) We actually want to do a lot ourselves, so masking and possibly priming and also applying the material in the simple rooms with not so high walls BUT we won’t manage 300 sqm timewise and strength-wise anyway, and with both painters, you can help as much as you want!!! and we liked both… what experience have you had with these two products?? or do you have other tips?? Actually, we wanted clay plaster paint, but according to the painters, the other two materials are more durable and especially better for bathroom / kitchen / staircases etc., and once we are fully furnished, we can also glaze both as we like...