Plural of nouns

Hello all,
I am doing an academical research about the plural of nouns in romance
languages. I want to ask you if you could give me some references about the
morphemes for number in PIE and Latin, particularly on the following
questions:
1) The morpheme for number is always cumulative with the morphemes for
case and gender? For example, in Latin, the desinence for ablative feminine
plural is -is; if we compare to the desinence for accusative feminine
plural (-as), we might think the -s is for plural, the -i- is for
feminine/ablative, and the -a- is for feminine/accusative. But this
conclusion is wrong, as we can see by lots of other evidence. I wish to
know if PIE was like this or not, and if someone has analysed the noun
morphemes of Latin in some other way.
2) In French, the plural is marked by an -s, but only in writing; only the
article distinguishes between "le théâtre" and "les théâtres". In spoken
portuguese, sometimes we pronounce "os teatro" instead of "os tratros".
Does thins phenomenon occur in other romance languages, for example italian
"i teatro" instead of "i teatri", or roumanian "trei teatru" instead of
"trei teatre"?
Bye,
Bruno