I only played Stellaris off and on, but I went years without buying an expansion and always thought the new systems were complete and better than what they replaced when I returned. Breaking current saves is frustrating, so I guess you would need to delay an update if you had one you planned on returning to.
If you didn't know, you can roll back to older versions of steam games with some work. A few games have a built-in system, but most of the tile you have to manually replace files after redownloading the old versions.
The jokers are very specifically worded for things like this. Usually they will say they give a bonus if a hand contains a certain (scoring) pattern. A straight flush contains both a straight and a flush, so jokers specifying one such contain trigger will proc. Similarly a full house contains a pair, two pair, and three of a kind too.
Rarely, jokers will specify you must play a hand instead, which means it must exactly match what is listed as the scoring pattern (except I think royal flush is still a straight flush), similar to planet upgrades. To do list is one example, because otherwise something like a three of a kind roll would be strictly worse than a pair.
Two pair seems to require two different pairs in a scoring hand to proc, meaning it doesn't count four or five of a kind hands. Some people disagree with this ruling.