I try to give advise from experiences i have witnessed or delt with myself thats all.
Rule books might not advise it but i setup a nano for a friend, 15 gallons (UK) After much stress in lsing 3 pairs of clown.. (This is not a setup problem but we have a serious clown issue in the UK at the moment with the advent of Nemo and wholesalers sending out stock far too young). I advised my friend to not get a clown just yet. Let the tank settle with a small neon goby and a cleaner shrimp. They did this and alowed the tank to stabalise over a period of 3 months. During this time i searched our local shops to find decent batches of clowns. I eventually found a shop that had a real nice selection of small clowns (small but not too tiny to cope with life in a new tank). I got 2 myself (and they are in my main 100 gallon tank which i have posted photos here) Iinformed my friend that these clowns were far better at coping with a new tank. I got them for him and introduced them together. 4 months later they are doing great! the tank actually holds a staggering 2 clowns, 6 line wrasse, algea blenny, neon goby, catalyunna goby, cleanr shrimp and blood shrimp!
This is too much even by my recomendation but they dont listen. I even went over to do a water check and was confident that their water perameters would be very unstable.. I was astounded to find their water quality was better than mine. no phosphates, Nitrates below 5ppm (mine were 20ppm at the time). This was with them doing hardly any water changes whereas i did mine every 2 weeks!
Basically what im trying to say is that there is no hard and set rules. If you are really careful and monitor your water permaters closely then you should be ok.
As far as the clowns for your tank is concerned. I would say its ok.. but first just add alot of liverock, perhaps a shrimp to help with stabalisation of the tanks water peramters. Then, after a month or so, add the clowns together. This will stop the arguing but the next dilemma you will get is that adding the 2 fish together in a new tank will incrase the bioload by 200% Its vital during this time to watch for spikes closley in a tank of this size. Run a skimmer during this time and only feed sparingly for about a week to allow the bacteria to catchup.
If you choose your clowns wisely, nice looking and active fish that are lively and bold then you will have a good chance of success with them.
As i say, this is going on my own experiences, it has worked even if its not the normal proceedure
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