I coated mine in Matt and while I like how it brings out its details, it also seems somewhat “rough” now which I’m not sure I like… Will another, different type of coat (I also have gloss but may try to find satin) ruin it?
Nope, it won’t ruin it at all. Well, putting things over a gloss coat can be tricky, as the upper layer may not adhere to the slick, shiny gloss. But you can totally put another coat over the top of a matte or satin finish.
As SPark said, it is perfectly fine, in fact people in the model horse community do just that when working with pastels. You apply a matte sealer on between layers… not only does it protect each layer from the next, the ‘tooth’ of the matte sealer helps grip on to the next layer of pastels. And then when all is finished, you add your finishing sealer of choice.
Update: I’m impatient and despite like a billion google results, I still felt unclear about how interchangeable some things are, and what things are the same as other things but with different names… So I just tested some medium/varnish combinations on my own, then went ahead and did my statue. Fingers crossed!