It's something I have alway done mate I get at least an extra truss on each plant. I start my seed in February then move them to the green house after I think no risk if frost but still cover with fleece at nights til I think its safe.
In the pots that are sunken into the ground ( never any smaller that a builders bucket) I put about 4" / 100mm of compost and plant the tomato plant in that. As it grows I take the lower leaves off and add more compost, I keep doing this till the pot is full. The stem of the tomato plant sends more roots out as it grows so you end up with a bigger root s