Fideos pasta in my area is generally overpriced, so I suggest buying angel hair pasta and breaking that up into the appropriate size. Spaghetti does work, but I find it is a bit too thick to give it the same texture is fideos pasta.
It is fun to mix various pasta shapes for texture if you are making a soup. I like to use a combination of fideos pasta and conchiglie to create a good look.
Just as a note, it can be a bit difficult to make sure all the pasta is cooked correctly if you are using pasta of different shapes, so it may take some practice to get the timing right.