spacing
The spacing is too much; I want some space between the equations, but not as much space as of the following. I would appreciate if someone could provide me with a coherent answer.
\documentclass[11pt,a4paper]{article}
\usepackage{blindtext}
\usepackage{mathtools}
\DeclareMathOperator{\sgn}{sgn}
\begin{document}
\begin{flalign*}
&+(+a)=+a,& &-(+a)=-a&\\
&+(-a)=-a,& &-(-a)=+a&
\end{flalign*}
\end{document}
I don't really get the question so I hope this is what you wanted. If you include a full document (such that we copy paste and see the problem on our systems) things are much more easier.
Here, you can change the default setting within a scope but your block style had a node distance which was resetting every time it is issued. I've made it 2mm such that we can see the difference easier.
\documentclass[tikz]{standalone}
\usetikzlibrary{arrows,shapes.geometric,positioning}
\begin{document}
\begin{tikzpicture}[decision/.style={diamond, draw, text width=4.5em, text badly centered, node distance=3.5cm, inner sep=0pt},
block/.style ={rectangle, draw, text width=6em, text centered, rounded corners, minimum height=4em, minimum height=2em},
cloud/.style ={draw, ellipse, minimum height=2em},
line/.style ={draw,-latex'},
node distance = 1cm,
auto]
\node [block] (1st) {1st};
\node [block, right= of 1st] (2nd1) {2nd1};
\begin{scope}[node distance=2mm and 10mm]%Here we change it for everything inside this scope
\node [block, above= of 2nd1] (2nd2) {2nd2};
\node [block, below= of 2nd1] (2nd3) {2nd3};
\node [block, right= of 2nd1] (3rd1) {3rd1};
\node [block, above= of 3rd1] (3rd2) {3rd2};
\node [block, above= of 3rd2] (3rd3) {3rd3};
\end{scope}
\node [block, below= of 3rd1] (3rd4) {3rd4};
\node [block, below= of 3rd4] (3rd5) {3rd5};
\path [line] (1st) -- (2nd1);
\path [line] (2nd1) -- (2nd2);
\path [line] (2nd1) -- (2nd3);
\path [line] (2nd2) -- (3rd3);
\path [line] (2nd1) -- (3rd1);
\path [line] (1st) -- (2nd1);
\end{tikzpicture}
\end{document}
At a technical level, detecting whether \SI is used inside display math is already implemented for the detect-display-math option. Thus adding functionality to make a choice of spacing based on whether \SI is used inside display math or elsewhere is quite feasible. Presumably this would require splitting the number-unit-product into two parts, with an alias meaning that the existing option continues to work: as usual, suggestions for names would be welcome. My main concern with adding such an option would be that I've never seen this type of variability in printed material: I'd like to see an 'official' source for such an approach before adding it.
Best Answer
Not sure what will best meet your criterion of "not as much space" being taken between the two columns of equations. The following solution may fit the bill. It also results, incidentally, in better centering of the material -- which I take to be a criterion as well.