Honestly , a 10g should be fine for 2 clowns this size, as they grow bigger it may get tight at some point..and I have seen some really nicely kept nanos that size housing clowns a Nem ( small) and zoas etc. Only thing to consider is lighting and maturity of the tank for the Nem. Also good parameter maintenance , of course. As for different clowns.. I have very successfully kept pairs of each species .. Peacefully . While u can pair them together , size of the fish is the most contributing factor. It will be a good idea to trade one for a smaller fish.. They will pair in MOST cases.. But as stated earlier by others , there are no hard fast rules in this hobby. Except for the one where I'm unable to keep Magician Palys No matter what!
good luck!
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