There are indeed limitations of the TeXshade package on which the function msaPrettyPrint() is based (see also Section 7.7 of the package vignette). However, I think that 20 sequences with ~1,000 bp each should not be a problem. If possible, please send me your sequences and the output of sessionInfo() on your system. Please send the data privately to msa@bioinf.jku.at. If there is anything secret about the sequences, you can anonymize the names. Ok?