Majority aro keepers do not have gravel as the unfinished food or poo will be trapped in the gravel; in which it will become ammonia which is harmful for yr aro and other fishes.

If you have bottom dwellers such as loaches, catfish, etc., they may help you to clean up those unfinished food.

So in answering yr question, it all depends on your setup. Hope this may help out.

For yr 2nd question, your aro will not brush it's skin against the gravel unless you have other tankmates which yr aro might chase/attack them. Arowana usually swim at the top and middle portion of yr tank.

Cheers

p.s. You can find more info at:-
www.arowanaclub.com
www.arofanatics.com