I can give you examples of when I use the different grabs, but not really specific moves to use it against…
-after you hit with st.sh/low short xx jab hayate (sometimes you’re not in range for it though after the low short xx hayate)
-anything parried close that’s not cancelled
-after a close jab tick throw setup
-dash in -> sh.karakusa
-on your wakeup, or when pressuring another character’s wakeup (whiff something/do nothing)
-after parrying Yun’s shoulder or overhead kick, but not the hop punch
-jumping mk hit or block
-close st.strong tick setup
-close cr.forward tick setup
-close cr.short tick setup
-jab tick depending on range
-this is very risky, but if anticipating a Ken low forward that’s a bit inside its max range, Ken’s low forward will kara himself into your grab if timed right.
-st.short xx hayate -> cancel -> forward grab. Can do sh.karakusa, but forward seems better to me, especially for dexterity reasons. This is a sneaky setup I use fairly often that has a lot of success.
-on Ken, if he rolls a normal throw, instant uoh asap, will whiff, grab (timing is real tight on the uoh, if too slow, will hit REALLY meaty, in which case you can link a st.short)
-from st.short kara
-really probably the only time to use RH grab in a normal situation when another one wouldn’t be better
-when parrying divekicks, if you want to grab, it’s probably best to use this by default, unless you’re really watching some ranges
-if you hit with a deep jumping fierce/rh and really want a grab, you can rh.karakusa with a slight delay right upon landing and grab them right when they recover (of course grab used can vary with how late the fierce/rh hits, vs. what character, and how long of a delay you use)
Some of this is just general as it will vary depending on when the hits occur, what character you’re hitting, what move you’ve parried, and the distance you are when parrying it. Just try to always remember short karakusa is the quickest, but has the least amount of range, and rh karakusa takes the longest, but has the greatest range. If I have a question as to which one to use, I usually go with the forward grab by default.
Hope this helps.