Yes they can do that and indeed, nobody could stop them. Whether it would be sensible to do so, that's entirely a different question.

Of course they can use the pound but without a seat at the table, money supply, interest rates etc will be decided on the basis of what's good for rUK

Perhaps I'm over-simplifying, but how can you be financially independent without control of your own currency/interest rates?

No country has all that much control over such things. Zimbabwe for instance let that stuff slide. After a while they had hyperinflation, and the only way to get rid of that was to junk their own currency entirely and switch to the US dollar (for which they neither sought, nor were given, permission).

If the Scots choose to use another country's currency as legal tender, and to pay their doctors and soldiers with, they wouldn't be able to issue it, but they would still get some advantages (of a mixed blessing variety). Their companies wouldn't have to hedge that currency (a big advantage if, say, 80% of your trade goes through one partner) and cross border transaction costs would be minimal.

Also, all the household and corporate debt in the country wouldn't have to be hurriedly converted - Scottish mortgage holders wouldn't need to switch banks to avoid having their debt denominated in a currency they no longer directly use. Foreign currency mortgages were a big factor in a couple of Eastern European financial busts recently.

And then there's government borrowing. If Scotland were to print its own currency - even with a peg to the pound or Euro - bond markets would be rough for them. They can either borrow under UK law in their own currency, with a hefty margin added if the markets expect them to devalue in the medium term. Or worse, they can borrow in a foreign currency, in which case a devaluation anywhere down the line would be ruinously expensive. If you don't like Tory austerity, wait till you experience it with the bond market's gun to your head.

It's a mark of low confidence in themselves that they want to wear the straight-jacket of currency union, but such unshakeable restraints are usually conducive to trust (this offer excludes half the Eurozone).

if Scotland vote to leave the union then it would be better for them to take the €uro as their currency
as they would have to re-apply to join the EU, if they desired, and a requirement for joining the EU would be the €uro ...

which is one of the main arguments. They (the yes people)say they will be allowed to re-join and not have to have the Euro (as they know saying scotland will move to the Euro is a vote loser) whereas others (including the European President) says they will have to apply and IF accepted would have to take it.

The other issue is that if they use the pound (which they can do) they will not have a Central Bank and one of the EU rules for Banks and other large Financial instituitions is that they must have their Head Office in a Country where they do substancial business AND has a Central Bank so immediatly many of their financial institutions would ahve to relocate and take all of the jobs (and tax) away

As far as I'm concerned if Scotland vote to leave Britain then they have to have their own currency or try and join Euro.