Hi Gathorne,
It's not unusual to have lemons at different stages of developement on the same tree, which I think is an added bonus.
I would think that, rather than treating them like tomato plants (keeping them to about 4 trusses) allow the flowers to develope (or not) then see how many of the flowers have set fruit, then thin out some of the fruits to keep it in proportion to the size of the tree, making sure the branches dont become over-laiden.To coin a phrase : dont count your chickens before they've hatched. Good Luck Tbird+