Pea gravel is fine for Clown Loaches, they love to dig in it. The 55g will work for three loaches for a bit(the bigger the tank the better, it's sometimes hard to afford a larger tank later) but eventually you will need a much larger tank. Mine went to a 75g in the first year and there were only two of them( the third one kept dieing) . The larger the group the more you see their natural actions and they were named "clowns" for a reason

I really wouldn't mix Corys and clowns. The clowns won't hurt the corys but they are such vorocious eaters the Corys will starve.
Sand tanks are really easy to take care of you just swish the siphon over the top and all the debris comes up because it doesn't get down in it like it does the gravel. BUT...clowns are such big poo'ers you would see it laying around on the sand . Don't get me wrong they would love it.
Personally I don't think crushed coral is a good idea as it could be too sharp for them.
Please do all the research you can before buying them as they are a vey long lived fish and deserve the best we can give them. A friend has one that turned 21 yrs last year. The link given above is a great place to start.
Hope that helps.
Carol