What contact tip do i need for welding?

i have a clarke 100en welder and 0.8mm gasless wire but there is a 0.6mm contact tip on the welder my question is,although the wire went through it will not feed will i have to change tip to 0.8,and will gasless wire work with gas,the other bit is can i use a larger contact tip e.g.1.0mm with this wire
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