Widely linear (WL) processing has been of great interest these last two decades for multiuser (MUI) interference mitigation in radiocommunications networks using rectilinear (R) or quasi-rectilinear (QR) signals in particular. Despite numerous papers on the subject, this topic remains of interest for several current applications which use R or QR signals such as anti-collisions in Radio Frequency Identification (RFID) or satellite-AIS systems, for grant free massive access in NB-IOT networks, for multipaths mitigation in the Control and Non Payload Communications (CNPC) link of Unmanned Aerial Vehicle (UAV) or FBMC-OQAM networks. In this context, the purpose of this paper, and the companion paper [50], is to get new insights into WL MMSE receivers for communication networks using R or QR signals, aiming in particular at discarding the need to estimate the MUI channels for their implementation, always cumbersome in practice, while keeping their MMSE optimality. For this purpose, several WL MMSE receivers, with and without any structure constraint, exploiting or not the cyclostationarity properties of the signals, are considered in this paper and [50] and their performance are computed and compared, both analytically and by computer simulations, for R and QR signals in the absence and in the presence of one MUI. Several new enlightening results are obtained, for two-input WL MMSE receiver in this paper, and three-input WL MMSE receivers in [50], showing in particular the non-equivalence of R and QR signals for WL MMSE receivers. Moreover, the conditions under which WL MMSE receivers discard the need to estimate the MUI channels while preserving MMSE optimality are given. These results are very useful for implementation issues optimization in particular.