două is feminine and doi is masculine. un hamster, doi hamsteri because the word hamster is masculine. In Romanian, nouns have genders: feminine, masculine, neutral (for neutral, the singular is masculine, while the plural is feminine, for instance wardrobe: un dulap (masculine), două dulapuri (feminine))
Yes, there is a difference. "Hamsteri" ends in a faint "i" sound, but the main accent of this word is on the first syllable. It is also worth mentioning this word is separated into syllables ham-steri, and in the syllable "steri", the accent is on "e", not on "i". When you have the articulated word "hamsterii" (meaning "the hamsters"), the accent is going to be on "i". I hope this was clear enough.