If I understand your problem right it might be a bit of confusion over how nim stores data. A big advantage of tuples is that their size is known at compile time which means that you can use them anywhere where a built in value type like an int could be used - within composite types, in the data or bss segment, the stack and so on. This however does mean that you can't create new tuple sizes at run time.
The thing all these locations have in comman is that the size of the data has to be known at compile time to use them. Anything with dynamic size has to be placed onto the heap which does a certain amount of householding to track which blocks of memory are taken or freed which in turn does impose significant runtime costs. Long story short: If you want a dynamic length sequence of (int, string) tuples you probably want to use seq[(int, string)] or something equivalent using a seq of objects.
