I do not want a big tree. The fork of the 2 branches is at 10 cm from the soil surface and the 1st thorn is 4.5 cm from the fork. At what height do you think I should cut?
Any specific season or throughout the year?
I would consider cutting just above the 1st leaf. It should shoot new buds further down the trunk if you cut there but will not bud if you leave the trunks long. Given the species and the current shape I think you will have to settle for at least a medium sized tree - or you could get rid of it and put your effort into something that is more likely to give a good small tree.
i would prune citrus in late spring or summer when they are growing. It will be at least partially dormant through winter so probably will not respond to pruning now.
Can you graft say cumquat on to lemon or does it need to be on the trifoliate base?
As far as I know almost all citrus are compatible for grafting so you should be able to graft cumquat on to lemon. I seem to recall that one of the lemons (Eureka?) will not grow on trifoliata stock. I read somewhere that the rootstock does have a more significant affect on fruit quality in citrus than other fruits so they do not use normal orange or lemon for rootstocks. Trifoliata is used because it is tougher, more cold resistant and tolerates a wider range of soils but several other rootstocks are used or have been used in the past. I have found that many citrus species grow well from cuttings.
