If you need to have a " within your FString, use the following:
FString MyString = "Here I need there to be a \" symbol."; // Backslash before the "
Yes, you can actually reserve memory for FStrings. Since it is basically just an array of TCHAR, it works the same. And it can be really helpful to prevent too much dynamic runtime memory allocation.
But: it does have a downside. Using for example the "+"-operator may still allocate memory on the heap for each appending. I haven't yet tested much, since TStringBuilder seems to be the better solution to do everything on the stack and then allocate memory on the heap once for the final FString.
FString MyString;
MyString.Reserve(128);