I haven't done knit yet, but I'm about to as well so would love some advice.
As for towels, what I do is the following.
Hoop a piece of water soluable stabiliser. Place a template of the design on the towel and pin in place. Use a quick spray of 505 glue on the stabiliser and stick the towel to it. Centre the design under the needle so it's just where you want it. Remove template. Slide another piece of water soluable stabiliser under the needle and sew. This will ensure that the stitches don't sink into the towelling. Rinse several times after to remove WSS. The towel may feel a little stiff but after a few washings the embroidery will be lovely and soft.
For the knit, I've bought some Floriani stabiliser that irons onto knit to give it more body. I'm going to try that before sticking onto the stabiliser.

Did you view the video that came with your machine? Brother is usually pretty good about covering topics like this.

Always do test sewouts with different combinations to find the right combination...good practice too.

Ironically, I was watching the cd for my singer CE200 this afternoon to test the hookup of my cable to dvd/cd player to tv. What they said for knits is to use an iron-on like fuse-a-knit for the bottom stabilizer, and water soluable on the top. You use the temporary spray adhesive on the bottom and press the knit to that..do not hoop the knit because it distorts and leaves a permanent mark. It also said that a flat knit works better than a waffle or textured weave. If you have a textured weave, it's better to do the design on a separate fabric and then tack it to the article to be emboidered.

On towels, it said that a velour weave works better than a looped weave. Sometimes you have to play around with the different water soluables if you have a looped weave. Some people will use tear-away and temporary adhesive that or use sticky back, cut a window in it, peel the paper back for a window and stick the towel to that with the WSS on top...you have to have a topper regardless or the stitches bury themselves in the terry.

I don't like to embroidery on knit blankets unless I make the blanket. You have to put something on it to stabilize it & it shows afterwards. Totally Stable may work best & remove what you can afterwards. You can also use the tricot interfacing & heat it afterwards & you can lift it some to remove part of it. I make my blankets then use a corner piece to embroidery on. I sew it across the straight edge & finish the other edges as I finish the edges of the blanket. I too never hoop knit or towels. I stick them to stabilizer too.