First/Second batch

I am a newbie at making wine and have just started my first "real" brew. However, I have noticed one or two potential problems that I hope someone might help with.
Firstly, I have a large fermenting buncket which takes a bung and airlock. I mixed up my primary stage last night and placed the bucket in a room to work for a week or so. When I checked this morning, I noticed a "musty smell" in the room. This suggests to me that the some part of the buncket isn't air tight, there is an airlock bubbler with water in it but the smell seems to be escaping. Could this damage my batch?

Secondly, and its simialr to the first, I bought a large carboy but noticed the bung I bought doesn't quite fit it exactly, i.e it most likely will not be air tight. Does the seconday fermentation phase require the carboy to be totally air tight? If so, any suggestions?

Thanks and sorry for the stupid questions.

welcome.

The musty smell is from the Co2 escaping from your fermentation. This is normal.

For the carboy, go ahead and get the proper size stopper. It will help to eliminate any more problems in near future.