Who knows, it might be better without him. Big Finish has done well over a hundred Torchwood audio adventures, many of them without Jack, some with only one or two characters from the TV series, and overall the Torchwood range is probably the most consistently good work Big Finish has done. If you haven't heard any stories with, say, Andy and Norton Folgate, or Mr. Colchester and Ace (yes, that Ace), you've missed out.
Well, the Silurians changed appearance a few times, and some tie-in sources from the BBC apparently claim the Sea Devils are an aquatic species of Silurian, so... probably not, would be my guess.