Hi,
I would for sure use some existing media or mulm/substrate from a mature tank to help. toss a few sponges in that tank NOW and they will also colonize a bit...
Depends on how many fish you cycle with, your water change schedule, etc. Honestly, your bio-bed will adjust to the size of the load. So if you cycle with guppies, and then toss in a full grown dovi, then I would say you will not have a large enough bio filter
You can feed more heavily in the mature tank which will spike the nh4 and then some more bio bacteria. This will help seed the extra sponges you tossed in there.
I don't know a thing about rays or their needs, but a 'better' filtration system is all in the eyes of the user and what works for their configuration. I will say that sumps and wet/drys are excellent filters for fish only tanks as they provide an excellent bio filter and usually a bit more 02 than a closed loop canister.
Based on the size of the fish and their feeding habits, you may want a sump wet/dry in addition to a large mechanical type filter.
Or I had a closed loop with a large Nu-clear mech canister and a Nu-clear bio-media canister in a closed loop config on my 180 and worked fine. I do not however keep large fish...
Many ways to go here for filtration.
Hope this helps. Sorry I can't give you a definite timeframe as too many variables....
I have done sumps and closed loops and both worked well.