Anything I can pass on to you about raising mercs can pretty much be found in the last link I posted above as that link and the connecting histories are my personal experiences. The first two links (to our new keepers journals) include some of my recent, uh, coaching
.
I have not experimented with any corals with the mercs and only a little with the hummelincki (my diurnal octo) so I can't give any first hand experience on what to avoid
but I would reconsider using a frag tank if you have recent cuttings as the water is likely to have a lot of stinging cells. Additionally, if you need to have a light on the frag tank for the corals, you will likely never see your octo. My best successes with interaction with the mercs has been with a red light only tank.
In a conch shell, I can tell you that they find barnacles and live rock acceptable homes, they are very shy and only a few will interact regularly. The twenty is a good size. You need a minimum of octo-proofing (lowering the water level and a loose fitting top seem to be all that is needed for this species ONLY). I recommend using a red velum (either on the light it self or over the transparent part of the top) to change your lighting (LED's work but the velum filtered light seems to be antecdotally better), leave the red light on 24/7, feed them live fiddlers several times a week and attempt to stick feed (one of our newest keepers found that a straw works very well) a
small shrimp of some kind daily. Lastly, keeping them where they can observe you on a regular schedule but where there is not a lot of fast movement and nightly taking the time to sit in front of what will sometimes seem to be an empty tank will likely give you the most enjoyment from your individuals personality.
to TONMO. 
