It has been demonstrated that silicon pore optics can serve as the new technology for building the next generation of X-ray telescopes for astronomical missions. In order to build up an optic in Wolter-I configuration, the high performance pore optics (HPO) have to be co-aligned and integrated into pairs, forming so-called X-ray optical units (XOU). The stringent co-alignment requirements for a 50 m focal length telescope like XEUS (e.g. 1 arcsecond between parabolic and hyperbolic HPO) demand holistic alignment concepts, which integrate the metrology, the fixation and the performance verification. The application in space and the resulting thermal requirements in combination with launch loads and other mechanical restrictions must also be considered. Finite element modelling of different fixation mechanisms and XOU configurations allow one both to assess difficulties at an early stage and to validate solution strategies. This paper reports on the concepts, which have been developed. The most promising candidate has been selected to build a form fit function model. The experimental set-up to align the HPOs, the required metrology and first results of the performance verification at test facilities will be shown and discussed.