'bristolsparks' putting two 6 bottle kits, in other words 10 litres, in a 25 litre FV IMHO is too much head space, you need a smaller FV or use two DJs.
The use of an airlock in a 25 FV depends on how volcanic the primary fermentation is and whether it can give good CO2 cover quickly enough and keep it covered. As has been said here, relying on a FV without an airlock depends on how tight the lid is, I make The Range kits which state to put on the lid but not tightly. I have one FV which if I forget to ease the lid there is a 'POP' when the gas pressure is too much, but it never blows the lid off enough to cause a problem.
When using a 25 litre FV you have to remember that the volume of gas produced is 5 times that of an ordinary 5 litre DJ, so a normal airlock will not be enough. My personal choice is the Hamilton Baird 'Major' only £1-2 each depending where you buy. a cheaper alternative is a length of syphon hose into a bottle of sterilising solution, probably safer when you have a particularly volcanic mix.