Get them in now as long as the ground isn't frozen or completely waterlogged (if the hole fills up with water after you've dug it, that's not good). I plant my trees with manure, a handful of bonemeal and a handful of growmore. Having said that, they've been much bigger trees - two year old fruit trees which are about 6 foot high. I've got 700 hedging trees to go in in March. They'll be babies, a bit bigger than yours but not huge, they're going to go in with Rootgrow which has the mycorrhizae (?) fungus to promote root growth - I think I can just dip the root in and then plant. Maybe I've just fallen for some marketing...
Remember as well to guard your babies from rabbits and to keep the base clear of weeds. As for spacing, I don't know - it depends how big you want them to get? Hedging plants are way less than a metre, orchard trees several metres.