Now I know there isn't exactly hard evidence of gambesons used by the vikings (which is what I'm most interested in) but I'd still like to know from any reenactors out there what is the best amount of layers for comfort while not making it too bulky or hot. Thanks in advance.

For reenactment use, so no full contact fighting, especially if it's going under the mail, you should wear undertunic, tunic and light padding (although as you say there is no evidence for vikings using it, it is useful and more comfortable). Light padding for under the mail should be rather light, two layers of durable linen and between them one layer of thicker wool or cotton should be enough.

I'm not sure. I don't fight in bohurt. It depends on the material and thickness of the layers. Felt is I think the best for filling a gambeson for full contact, and felt comes in many different varieties. I guess the overall thickness should be at least a 1.5cm.