you don’t need to use trapcode particular to generate the particles – you can use sprites. just animate your balloon in another comp, render it out, and feed this into particular by selecting ‘Custom’ in your particles. you can set this also to select a random place in time so they aren’t all moving the same…
as for making them interact with eachother – maybe you should use particular for the main clump of balloons – then manually animate a few balloons colliding so it looks a little more real…