Looking at the native ranges of the lumpfish, topknot, common grey sea slug, and sarcastic fringehead (which seems to prefer slightly warmer and more moderate temperatures), it looks like average temps range (for the southernmost part of the range where they're still commonly found) from ~45-55F in the cold months and 55-65F in the hot months.
Assuming you're not looking to replicate a cold season and warm season, I'd guess you would probably want to keep the tank around 55-60F. Personally, I'd aim for 57-58F. If you are looking to replicate a cold season and warm season, I'd personally aim for 52-53F from December through May and 58-60F from June through November, probably taking four weeks to transition between the two temps (i.e. take the last two weeks of Nov. and the first two weeks of Dec. to drop the temp from 58F to 53F, and take the last two weeks of May and the first two weeks of June to raise it back from 53F to 58F).
If you don't end up keeping the sarcastic fringehead, I'd personally do probably 48-50F for the cold season and 59-61F for the warm season. That said, from what I can find, all of the fish should do fine with the temps listed in the paragraph above this one.
I'm not sure on the stalked jellyfish (taxonomic class Staurozoa), as the needs depend on the species.
All of this said, though, you want to make sure that all of your tank's inhabitants to be can get along with each other and not hurt each other (I have no idea if the stalked jellyfish would sting and hurt your fish or not, but that's something you would want to look into). Additionally, with the common grey sea slugs (assuming you're meaning Aeolidia papillosa), you need to keep in mind that these guys eat sea anemones as their primary food source, so to really keep these guys for any length of time you'll need to either regularly buy more anemones, or you'll need to culture your own (probably in a separate tank).
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