Here is a version of \HRule that allows you to adjust the gap above/below with an optional argument:
\documentclass{article}
\newcommand{\HRule}[1][\medskipamount]{\par
\vspace*{\dimexpr-\parskip-\baselineskip+#1}
\noindent\rule{\linewidth}{0.2mm}\par
\vspace*{\dimexpr-\parskip-.5\baselineskip+#1}}
\begin{document}
\noindent
\begin{minipage}[t]{.5\linewidth}
Here is some regular text.
\HRule
Here is some regular text.
\end{minipage}%
\begin{minipage}[t]{.5\linewidth}
Here is some regular text.
\HRule[0pt]
Here is some regular text.
\end{minipage}
\end{document}
The default gap (shown on the left) is \medskipamount (6pt plus 2pt minus 2pt), but you can adjust that. For example, on the right a gap of 0pt is set.
Since not all packages are given, I can only guess about packages.
It is a better method to change \arrayrulewidth within a group such that it does not interfere with other tables environments. Fortunately you are using a \newenvironment already, so it is quite comfortable to place it there. For better handling, it would be better to provide an (optional) argument to the mycustomtable such that the array rule width can be adapted without changing the code, but that is up to the OP...
\documentclass{scrbook}
\usepackage{tabu}%
\usepackage{longtable}
\begin{document}
\taburulecolor{title}
\arrayrulewidth=1pt
\setlength{\arrayrulewidth}{1pt} % Definiert die Liniendicke von Trennlinien in den array- und tabular-Umgebungen.
%referenced by btable
\newenvironment{mycustomtable}
{
% Lets overexaggerate a little bit ;-)
\setlength{\arrayrulewidth}{5pt}
\begin{center}
\def\arraystretch{1} %Seems to be better than to use %\renewcommand{\baselinestretch}{1} % Zeilenabstand
\tabulinesep=1.2mm % cell padding according to http://mirror.ctan.org/macros/latex/contrib/tabu/tabu.pdf
\begin{longtabu}[l]
}
{
\end{longtabu}
%\renewcommand{\baselinestretch}{1.5}% Zeilenabstand
\end{center}
}
\begin{mycustomtable}{llll}
\hline
This & is & a & table \tabularnewline
\hline
\end{mycustomtable}
\begin{tabular}{llll}
\hline
This & is & a & table \tabularnewline
\hline
\end{tabular}
\end{document}
In order to compare with a standard table, I have added a tabular environment too.
Best Answer
The latex syntax is
\rule
not\hrule
but if you want to use\hrule
thenshould work. The latex syntax would be