Mattt, yes I used DT's from the bottle to start the first culture and yes, I use one drop of liquid fertilizer meant for plants for 2 litres of water. I don't know how the discs work as I've never had one.
The only reason for doing things this way is that in Canada here, I don't know of sources for the culture starters or food and to get from the US is not always possible as many companies don't ship to Canada and the cost is prohibitive taking into account buying shipping and conversion from US funds to Canadian Funds.
Buying DT's was more than 4 times as expensive, plus shipping, compared to someone buying it in the US.
The fertilizer better be safe as people use it on food plants as well and we eat the produce without harm to us. (at least none that we know of)
At the moment, I feed the cultures to my brine shrimp which in turn feed my tanks.