Welcome to AquatiumAdvice.com seadonkey....
The 20
gal would do fine for the clowns. If you post the L"W"H", the true volume can be determined for sure.
As far as equipment needed, you will need a sand susbstrate (oolitic aragonite or southdown), heater, salts,
PH's and a simple strip light as it's just to be an
FO tank. You will also need to decide some type of filtration sysstem in the absence of the
LR.
FWIW, a
FOWLR would be easier (although more $$) to mainatin and keep healthy. On a 20
gal without rock, you may need a
HOB, canister or some other type of filter to help keep the water clean. You can also plumb the 20
gal and use the 10
gal as a >>
sump<< which would be much better than the first two filters mentioned or possibley a planted >>
refugium<<.
Other equipment will be a hydrometer or refractometer for measuring
SG or salinity respectivelyand the various test kits for cycling/water quality. They will be ammonia, nitrite and nitrate as well as
ph for after the cycle. I would suggest reading the article of >>
fishless cycling<< to get the tank's biological filter started.
One of the best books you can get that will help you with alot of this is
The Conscientious Marine Aquarist by Robert M. Fenner and Christopher Turk which can be found in the
book section.
That should at least get you started...
Cheers
Steve