Can FASTA files have nucleotide and protein sequences within them; or must they only have 1 type? Can FASTA files have nucleotide and protein sequences within them; or must they only have 1 type? For example, a FASTA file has 2 sequences. Can the first one encode amino acids while the second one encodes bases?

Thank you
While there's nothing stopping anyone from doing that with the FASTA format (after all, it's just a text file with '>' defining header lines), I don't know of any software that would support such a file structure. At best, it would interpret the nucleotide sequences as protein sequences (A/C/G/T are all valid 1-letter protein codes).

A better question to ask would be "Does ultra-cool-bioinformatics-tool X support combined nucleotide and protein sequences in the same FASTA file?" In which case the answer would most likely be, "No."