I can't find any resource anywhere that tells me how I'm supposed to knit a stuffed animal limb onto the body part.
Are you knitting the legs on or sewing them on? I usually use an overcast sewing stitch to sew parts on for dolls/animals. Use the same yarn as you worked with and take small stitches and they won't show much.
You can try different things on the mouth parts and see what looks best. You can always take out what you did and try again, just be careful not to cut your finished work in the process. The overcast stitch might work all right for that too.
I've not sewn felt on that I can remember but the overcast might work for that too. Or, depending on where it is you might use regular sewing thread and sew inside the edges of the felt a little with just running stitches in a color that matches the felt.
You don't have to use the same type of seams for everything. Use what works and like I said you get more than one try if you need it.