I transferred about 5 old
GW paints (the old mega paint set from about 8 years ago that I'd never used) to Vallejo dropper bottles which I bought from a
UK modelling site. They are about 30% larger than the old
GW paint pots so had plenty of air. When transferring I added a little water and liquitex medium to them as I paint with thinned paints anyway so it didn't bother me.
One thing I would say is a good idea is to add an agitator to the bottles. I used lava beads after reading legoburners great investigation here:
http://www.dakkadakka.com/dakkaforum/posts/list/240098.page
I love using dropper bottles and though you waste a little paint from each pot it's far better than letting it all get gunked up in my opinion.