This raises questions I knew we'd need to face eventually, so I might as well address the broad range of issues raised rather than just the specific matter of an agreement with Antica.
What follows is a series of economic ramblings; followed by possible solutions to pick from. Feel free to skip the ramblings.
1. Should we sign economic agreements with non-Small-Commonwealth nations full stop?
That is, are we still on a mission to get as many nations into the SC as possible? If so, then we shouldn't sign economic agreements with non-SC nations. If not, then we can.
2. Should we institute fixed currency exchange, and if so, who runs it?
As put in the SCUE treaty, we are not allowed to delete currency from circulation. Handling fixed currency exchange, then, means that normally speaking each government would agree to trade currency at a certain ratio; and would hold reserves of the other nations currency.
Example - if there were an agreement between Shireroth and Gralus (and no SCUE), we might say 1

= 2 GELT.
Gralus would give Shireroth 5000 GELT; and Shireroth would give Gralus 2500

. Then, if a Shirithian wanted GELT, they'd go up to MiniTrade and ask for GELT, and MiniTrade would give take their

and give them GELT; and vice versa; and the same for Gralus.
Now, with the SCUE, this gets more interesting.
Which government holds the reserves?
Say, for example, Shireroth agrees to do it - then all the SCUE people that want Antican money would go to Shireroth and give Shireroth SCUE currency. But all the Shirithians who wanted to cash in Antican money would also go to Shireroth and give Shireroth Antican money and expect erb. If Antica turns out to be fairly generous with their money (or a rich Antican like Phineas decides to ditch Antica, and just be Benkern in Shireroth, and tries to cash in all their Antican money), then Shireroth might have to pay out a lot of erb, and go bankrupt.
Alternatively, there could be a central fund, but who would initially contribute to said fund? Do we make everyone (even if Nelaga, Natopia and Stormark hate Antica (for examples sake) and don't want their money)?
3. Is the other currency a suitable one to have a fixed exchange rate with?
Mainly, this involves two issues: Do they have a set amount of money, or do they just keep printing more?
If they have a set amount of money, is most of it in circulation?
If the answers to both are yes, the currency is suitable. If the answers to either are no, then we have problems.
In Antica's case, they appear to have a set amount, of which a bazillion is held in the Treasury. If they keep putting more of this out over time, it really messes with exchange rates. If we set a fixed exchange rate based on the amount of money an ordinary citizen has right now, we'd be looking at 1 Antican:50 SCUE. But since they have a bazillion in reserves, they could put an extra 1000 out, and suddenly there's someone demandind 50 000 SCUE off us! Problem, clearly.
Alternatively, if we set a fixed exchange rate based on the amount of money they have in total, we're looking at ~ 200 Antican = 1 SCUE. Which makes all current Anticans phenomenally poorer than any Shirithian has ever been.
To a lesser degree there are tax questions, but if they work on the assumption that any SCUE citizen holding non-SCUE money can't be taxed, that resolves that.
4. Finally, how to set the rate.
I've written on this before.
MICEO had a kind of neat system but nobody uses that much anymore ...
Ok, I've given you problems, now solutions. I have
5 Plausible Solutions
Solution 1: No
The Small Commonwealth United Economy NEVER agrees to currency exchange. They have to use our currency or nothing.
Solution 2: Free Market
The Small Commonwealth United Economy NEVER agrees to fixed currency exchange. However, we're happy to allow the market to work (ie let individuals trade currency at whatever rate they choose).
Solution 3: Free Market + Initial Help
The Small Commonwealth United Economy NEVER agrees to fixed currency exchange. However, we'll negotiate an initial exchange of reserves; and then we're happy to allow the market to work (ie let individuals and governments trade currency at whatever rate they choose).
Solution 4: Fixed Exchange Rates (simple and problematic)
We say "screw you, economics" and use a random number generator to set an exchange rate with anyone who asks. We suffer whatever economic woes result.
Solution 5: Fixed Exchange Rates (complex corollaries)
We decide upon an exchange rate suitable for this point in time, and exchange reserves. Within the treaty, we have the ability to re-evaluate the exchange rate when the reserves reach certain levels (possibly happening automatically, using complex formula). This should hopefully prevent any country going bankrupt just because another prints money.
My very heavy preference is for the Free Market option (Solution 2). If you let two individuals agree on an exchange rate, it not only creates a new industry (money changing), but also prevents the problems stemming from "which government" and problems stemming from fixed exchange rates causing bankruptcy. Admittedly, however, this will only work if there are rich dual citizens, or someone interested in managing such a thing. Solution 3 is a possible work around for this.