I've just taken control of Mae'var's (sp?) guild, and I'm doing the first task, meeting someone in Waukeen's Promanade.
As soon as I arrive Ceara appears and talks to Kelsey. Kelsey replies something along the lines of "I don't want whatever your selling." Then the game crashes. I went to the Slums instead of Waukeen's Promanade, and she doesn't show up. The only thing I can think of is dumping Kelsey somewhere, doing the quest, and getting him back.
Is this a known bug?

That's unusual... I'm not sure what the problem is, but you might have more luck at the mod's support forum: http://forums.pocketplane.net

No, that's not expected behavior of the mod. And you can't do that quest without Kelsey in the party since it's tied to him.

I'd suggest reinstalling Kelsey. Run Setup-Kelsey.exe and select reinstall.

If I reinstall would it be fine afterwards, or would I have to start again, chuck him out of my party and get him back, etc. ?

Heading to Waukeen's after being given the 1st Thieves' Guild quest automatically forwards the game timer to midnight, to ensure that it's night-time when you meet the guy you're supposed to meet. Most quests (including Kelsey's) are on timers, and so going to Waukeen's moved the clock forward and triggered Ceara's appearance--whereas going to any other area does not mess with the clock.

It's possible that Ceara's appearance messes with something that screws up the meeting with Ama, so the key is making sure the two events don't take place at the same time. Head to the Slums or somewhere, wait for Ceara, and then head to Waukeen's and do your Thiefy stuff.