Yes, you can take them together. We are lucky that B12 has such a low non-existant toxicity profile.

The DNA mutations are just barely now understood. And the body converts cobalamin in food back and forth to the more active forms as needed. This is all done with enzymes.

I have some links to the Kegg pathway... and I'll warn you that it is complex and difficult to understand. But the diagrams illustrate how various nutrients in the body are converted around.
The boxes with numbers are enzymes. These are made from messenger RNA from our DNA...and if the DNA has an error present then the enzymes fail. This is really complicated and hasn't made it down to the lay public or doctors much. That is why I gave you MTHFR.net... that is one of the better sites for understanding this if you want to...that is.

This is human digestion:
http://www.genome.jp/kegg-bin/show_pathway?hsa04977

http://www.genome.jp/dbget-bin/www_bget?md:M00122

If you look around on Kegg you'll find other biological metabolic systems that have been so far elucidated. I find it all rather humbling! LOL But it illustrates enough to make us all respect how chemically and genetically complex we are!

From what I have learned, the adenosyl form is favored by body builders, for muscle energy etc.

But I think for the most part, we should keep it as simple as possible, and only move on when things don't go as well as expected. I stick to the medical evidence, because that is how I was trained, and how I worked in my own profession.

Anecdotal stories are interesting, but often are highly personal and reflect potential but not solutions for everyone.

This is good that you are seeing improvements quickly. You may have to take B12 for life, because something caused your low levels initially. The digestive thing is puzzling, because low B12 often causes diarrhea and other problems. If you have low acid, the B6 in the complex may improve that. Low acid causes gas and bloating and diarrhea in some people. A supplement called Betaine HCl is often used to raise stomach acid for people with low acid levels. It may be helpful for you to look that up if your issues continue. The digestive results of low acid involve fermentation in the GI tract of undigested food, by bacteria.
You might find Kefir very helpful, to restore balance of good bacteria in the bowel.