If two ideas, for example, are similar, the implication is there really are two ideas - that have many points in common (though sometimes the points in common may be very context-specific).
If two ideas are the same, the implication is there's actually only one idea (albeit perhaps presented in two very different forms).
This distinction is commonly understood. Thus, for example...
1: John's and Peter's essays both make the same point.
2: John's and Peter's essays both make the same points.
...are both valid. In (1), the implication is one single point is made by both essays (either or both essays may make additional points which aren't in the other). In (2), the implication is more than one point is made in both essays.
Thus, there's nothing wrong with saying "We both have the same ideas" - it just means we have more than one idea in common. But no-one ever says ?"We both have two same ideas". If the meaning is there's one idea [which we've each thought of independently], it's "We both have the same idea".
If the intended meaning is we've each independently thought of "Idea A" and "Idea B", English doesn't really have a succinct unambiguous way of expressing the concept using the word same. You'd probably rephrase to something like "We have two ideas in common", or "We share two ideas".
That's at the semantic level. At the grammatical level, all I can say is standard usage is at least consistent - we never say ?"This is a same idea", or ?"Here are three same coins". If several things are the same, there's really only one of "it" (within the current "frame of reference"). That's why it's singular, and why it can't take the indefinite article (to speak of "an X" implies the existence of multiple X's).
silly fat cats (or) fat silly cats
Silly fat cats is more euphonious. Both are grammatically correct.
'Fat cats' idiomatically means rich people, or rich powerful people. So it could be that you are calling those rich people silly, as opposed to calling those silly cats plump.
funny fat cats (or) fat funny cats
The rhythm of these phrases is about the same, so either.
funny big fat cats (or) big fat funny cats
Big fat funny cats rolls off the tongue. It has much better cadence. So definitely big fat funny cats.
funny really big fat cats, or really big fat funny cats
Again, the cadence is the deciding factor - really big fat funny cats.
really in this situation means "very", and modifies the adjective which immediately follows, so you are right to move the adjective "big" along with it...
Compare
A really expensive black leather handbag/purse.
This bag is black, as well as being very expensive.
An expensive really black leather handbag/purse.
This bag is expensive, as well as being very black.
Best Answer
Best is used all the time when talking about two things or people.
"The best 1998 science fiction movie about people fornicating with chickens inside the Astrodome is Glubaba, which is way better than Raise your right arm.