Do we have to pick a side? Or an I walk along the line to 50?
AFAIK, further Romulan specific story missions are locked until you choose, and Doffs, also fleets might be a possibility too because your choice determines which faction's fleets you can join. AFAIK there is nothing else that gets locked.
Despite what Kwait said, we don't know yet, if there will be an RR category for Foundry, but I suspect there will.
Note that there are apparently 40 levels worth of Romulan content, so even though you have to choose your alliance at 10, it doesn't mean your Romulan has to do Fed or KDF content at that point.
With regard to the Foundry, it would be nice if they would add an option to flag your mission for a specific faction in the search function, just as an identifier, not a restriction. This is, of course, in the event that they don't split out Romulan content like Fed/KDF.