A small tsu (sokuon) geminates (doubles) the following consonant. In native vocabulary, only unvoiced consonants can be geminated. This includes the さ, た, か, and ぱ rows. A double n as in おんな is not really pronounced the same way as ＊おっな would be if it were a word.
In loanwords that require gemination of other consonants, N tends to use ン, M uses ン or ム, W uses ウ, and others use the -u form of their row. Some examples are:
- homemade → ホームメード
- comma → コンマ
- whistleblower → ホイッスルブロウワー
- role-playing game → ロウルプレイングゲーム
Some loanwords do use the sokuon where native words would not, such as:
The important thing is that these are all loanwords. You generally won't see such examples written in hiragana, though the sokuon is used the same way in either writing system.
You may also see the small tsu at the end of a word or sentence. There it represents a glottal stop, an abrupt ending.