만두 국 (with a space in between) is acceptable.
But as a compound word, 만둣국 is the correct "spelling".
There are proper rules in the making of Korean compound words. Maybe someone could give us a run-through.
In addition to what LiKenun said, this added ㅅ(시옷) may be viewed as the "-" hyphen in English compound words.
너무 - so, too.
In general, 너무 can be used in 2 ways:
a. As "so", an exagerated 아주, "very" - in sentences with positive tone.
너무 맛있어요 = It's so (very-very) delicious
☆ It is often misused as a synonym of 아주, esp. in speech Korean. But the risk is it may sometimes be misinterpreted by the listener as sarcasm (e.g. over complimenting someone)
b. As "too", meaning too much, excessively, over the limit - in sentence with negative undertone.
너무 매워요 - It's too spicy (more than one can stand)
Practically, yes, prescriptively, no. https://www.facebook.com/groups/1778985748843114/permalink/2059839977424355/