Generally, yes, and yes. The fewer non-words in the grid the better (that includes abbreviations, acronyms, initialisms, Roman numerals, prefixes, suffixes, letter runs and anything that isn't an actual word or name). Of course there are instances where a familiar abbreviation or initialism (FBI) is better than an obscure word (FUD). Unfamiliar abbreviations are likely to be disallowed (virtually every 3-letter string is an abbreviation of something). Plural abbreviations are particularly disliked, unless they are something you might say in a sentence ("Our college has lots of PHD's in its faculty" makes sense, but you'd never say or write SCI's).
