Thank you for the all your comments.
I shall not be adding anymore fish, but may I ask how many you would consider the max. The original store where I purchased the tank suggested I could have up to 20 small fish. Even I thought this was too much, however the store I have used most recently thought 12-14 so long as my water quality was fine, which it seems to be.
It may well be overstocking, however even when I only had half a dozen fish, I still had random deaths. I shall continue with my weekly water changes for the time being as my parameters are ok and I don't want my nitrates to build up. I shall however up my water testing to double check that there is no cycling going on.
As for the poop, my livebearers have always had a tendency for stringy poop, but usually greeny. The only time they have white poop is when I've put one of those white holiday blocks in whilst I've been away. May I ask what problem does 'whitish stringy poop' suggest?