Tents, to be linen or not to be linen.

[Forty-One]

But It also happens that ‘felt’ is just about the same word: pilleum, pillei, pilleus, and that perhaps pellibus may have meant felt also (a latin expert is needed here)

No. The words are completely unrelated. pellis is used for "skin" or "hide" or "leather" or "parchment" (as is corium). A pilleus is a hat made of felt or hair. It derives from Greek "pilos" (which means "wool", or maybe "felt") and Latin pilus which means hair. It is presumably nominalized from an adjective meaning "hairy" but none of my dictionaries explicitly say so.

sub pellibus means "under skins". It doesn't imply any other material - certainly not felt.