さっか is “sakka”, not “tsaka”. The small tsu “doubles” the next consonant when writing it in romaji, but in actuality, I think it represents a glottal stop (someone correct me on this).

Small tsu and small ya/yu/yo are used for totally different things, they just happen to each use small versions of the characters to represent something else. Small katakana ke（ヶ）is actually pronounced ka and used to show number of months, e.g. 三ヶ月 means “3 months” as opposed to 三月 meaning “March”. There are other small kana used for different reasons than these too.