Yes. 1 clown is more than enough for a 10g system, your nitrates prove this.
In freshwater you don't want to go about 20 nitrates, in saltwater with invertebrates you try not to exceed 10, 5 preferably.
I'd only do 1 clown to cut back on bioload (and possible over feeding)
Invertebrates will be fine.
I have a snowflake ocellaris
And a snail in my 10g tank
Nitrates stay in check.
In a reef you can't let nitrates get that high or you will have some unhappy corals.