The BN would be fine in there if it were on its own as the main/primary bottom feeder. It will grow to around 6" I think (someone can correct me if I'm wrong), so I would think it'd be ok.
The clowns though: really, you're looking at having a difficult time with them if you decide to keep them. They may look small now (and actually the smaller the more difficult to keep IME since they're a sensitive enough fish as it is without them being young as well), but they will grow quickly. The growth rates are very quick among macracanthus.
"Juveniles exhibit incredible growth rates. A doubling in size is discernible in a week, if sufficient food, adequate aeration and filtration are provided."
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http://www.dbs.nus.edu.sg/biodiversitii/bio/aquarium_more.html
It's true that once they hit the peak of their growth rate they slow down some and can live for a very long period of time (10+ years in the home, and 50 years or so in perfect conditions, i.e. their natural ecosystem). But, you're looking at a fish that can grow up to a foot in length in the indoor aquarium. That's a lot of fish:
i) for your filter to cope with
ii) to look after
iii) to physically accommodate
Clowns are also highly succeptible to ich: IME they can even get an outbreak from the stress of a necessary water change!
As botias are generally a highly sociable fish, particularly macracanthus, you are also increasing the chances of a more stresseful setup by having only two. You may find they will fight more often than normal, and one may end up being pestered to death.
These are all hypotheticals, but they are risks you needn't face. LFSs are usually quite happy to take back stock if you telephone ahead of a visit and speak to them about it.
That you felt a niggling feeling enough to ask whether the tank was appropriately stocked should be a sign that you know deep down it's not great for your pets, and it'll be easier in the short term to do this than to face handing them over 6 months down the line when you've grown attached to them.
