I liked the 18-55 as a general purpose single dive lens. It has close focusing capability and allows limited WA and SM using a +5 or +10 diopter. Most of us did this because there wasn't much else to use and it worked okay. Yet the lens isn't known for stellar IQ: some copies were terrible, while others( like mine) quite sharp for a kit lens.
I think the 18-55 will work fine in the N7" dome, but you may be better off using the Zeiss 16-70 zoom over the S18-55 kit. Nauticam now makes a zoom gear for it and the 7" dome. I've considered going this route myself, along with modifying my Port 72 swing mount to fit the 7" dome metal shades, but this would be no easy task and the Z16-70 is very expensive ($750-900). You are trying to save money! However, without mounting a diopter for macro, the 18-55 is not a very useful lens underwater as mentioned above, especially when you have the S10-18. This is true for the Z16-70 for the same reasons especially since it doesn't have as close a minimum focusing distance as the 18-55. Middle ground zooms seem to work best as single dive lens options if you can use both ends, which means adapting a macro mount to the port, not very practical or easy to do on a dome, but readily available for flat Port 72 and the 4" semi-dome ports.
Should you want to use your S18-55 in a separate port, flat Port 72 will give you maximum magnification for SM, but less AOV and increased CA and soft corners on the WA end. The semi-dome 4" port solved the problems on the wide end, but lost the 25% magnification on the macro. I think most NEX photographers chose the Port 72 for the added magnification, since 55mm is rather short to begin with. Used Port 72's are fairly cheap and readily available now. 4" semi-domes don't exist on the used market (I've never seen one for sale). I may sell my Port 72 with a clip on macro adapter soon, since I bought the Nauticam Canon 60 macro port for my NEX 7 rig. I plan to move my 18-55 to my 4.33 dome (w/ swing mount adapted to it) and use the S20 with WA converter for WA (or other WA options that fit the 4.33) while keeping the 18-55 for a single dive lens with 25% less magnification on the macro side. It's wide end should perform better, however, in the 4.33 dome vs flat Port 72. Seems like us NEX users are always experimenting with what we've got. LOL.
Then there's the Zen NEX adapter allowing you to use S&S domes and macro ports which have wider possibilities for the future lens development.
No easy decisions when it comes to NEX housings and lens, but things are getting better.
Good luck and happy diving with your NEX6.