Translation:The potatoes go fantastically with the meat.
This is because the verb "passen" requires the preposition "zu" if you are talking about combining two or more items. Another example:
Dieser Pullover passt sehr gut zu meiner Hose. (i.e., the style and the colors are harmonious)
You can use "mit" if you use another verb, for example:
Das Fleisch mit den Kartoffeln schmeckt sehr gut.
No. There are rules in English where to put an adverb. And the common place is just behind the main verb. Exceptions are auxiliaries like "to be", which have the adverb in front of them.
And if the predicate consists of more than verb , the usual place is after the first verb form:
"I usually go ..."
"I am usually tired"
"I am usually going ..."