This looks more like a question about English terminology than about Italian language, and as such it would be off topic. Italian grammar only considers prefissi (at the beginning of a word), suffissi (at the end) and infissi (in the middle);1 apparently – but this should be asked about in an English-language site about general linguistics (such as https://linguistics.stackexchange.com/) – this corresponds in English to the distinction among prefixes, suffixes, and infixes.
As for “interfix”, I admit I never heard about it and more than one English dictionary don't include it, but apparently, if we trust Wikipedia, it is “a phoneme which is placed in between two morphemes and does not have a semantic meaning”. With this definition, none of your examples is an interfix, since those strings of letters: a) are not single phonemes; and, more importantly, b) do have a semantic meaning. Indeed, -er- in ameremo characterises the future tense, while -in- denotes a diminutive form (tavolo = “table”; tavolino = “small table”).
1 See for instance the article “affissazione” in G.L. Beccaria, ed., Dizionario di linguistica, Einaudi.