They're just planning ahead..it's what they do.

The quote is...

"There might be an issue with availability."

^{My emphasis}

It's not that there WILL be an issue...just the possibility of one.

In other words, it's clear that they know that Basher is involved in the proposed bank robbery and that there is the potential for something to go wrong.

So, they plan ahead and are ready to step in to retrieve Basher IF it goes wrong, which it does. If it doesn't..no harm done and they can move on.

If they didn't need Basher until after his bank heist was finished anyway, why was there any "issue with availability"?

Because if they don't step in if something does go wrong then Basher would be unavailable as he would be either in jail or prison...so "unavailable".

Basher has a tendency to work with "tossers"; unskilled criminals who are prone to mistakes. You might recall that he even referred to Rusty and Danny as "proper criminals"; criminals who plan to the Nth degree for any possible hiccup that may occur. As such, when Rusty mentions a possible issue with availability, he's referring to the fact that Basher is working with a team that doesn't plan jobs as thoroughly, and thus have a higher probability of getting caught.

Rusty didn't know the heist would fail, but he knew they had a good chance of getting caught. Enough so that it was worth being on the scene in disguise in case it went south.

I think you're making a false assumption, namely that when Rusty and Danny were having the conversation in question that he knew that Basher was planning on participating in a particular bank heist. He may have, but there's no specific evidence from the movie. I believe that it's possible that he simply knew that Basher was actively working with other criminals, which could limit his availability.

As for his impersonation of a detective, it is likely that Rusty, as a professional criminal, would have access to several different disguises and identities, as evidenced by the doctor impersonation later in 11 and the earthquake expert he pretends to be in 13. Having a law enforcement persona at the ready is only logical. From that point, obviously he found out about the bank heist at some point (otherwise, why would he be there at all?), and went to meet Basher upon his exit. Some time would have passed between the tripping of the alarm and the arrival of the police, so Rusty may have arrived after the squad cars. Seeing the ensuing police activity, he would reasonably assume that the heist went south, get together some ingredients for a small explosive, and wait until he had his opening.