Thanks for the pointers. Having read a bit more, Endlers might be a better way for us to go, being a smaller fish than guppies. We did see some tiny ones (presumably juveniles at around 10mm long) a few weeks back at the LFS (a beautiful black with a gold stripe), though I didn't realise what they were at the time. I'm not sure about adding females, 'pretty boys' as you put it lol might be the best way to go rather than dealing with tiny offspring.
Thanks for the tip - I didn't appreciate that snails had such an intolerance to a lower Ph - does that not exclude them for almost anyone with a high tech tank? We were planning on adding a couple of Zebra Nerites, so may have to rethink that.
Oh he loves the shrimp, as do I, they are truly fascinating to watch, so we'll definitely be having shrimp in one form or another. His attraction to guppies was just seeing them in the LFS. If you think through the eyes of a 7 year old that has never had a tank before, and you visit your LFS and see a tank full of guppies going crazy up and down the glass, next to a tank full of tetra hanging largely motionless in the water column, which one are you going to get excited about! I have explained to him that behaviour of the tetras will be different in his own tank, and he does love the look of Cardinals so they will likely be our mid-water fish of choice I think.
We may add a few Otos with some Cory Habrosus as I've read they shoal together in the wild, a different LFS near us has some lovely looking striped zebra Otos that I'd not seen before - I've only ever kept the Affinis type in the past.
There are too many nice fish, that's the problem - at only 60 litres we have to watch our stocking levels! We can handle a high stock in terms of filtration (the Eheim 2028 is rated at about 21x at 1260l/h - though I appreciated real world flow will be a fair bit lower, we should still be well over 10x) and we have the facility to auto-water change 25% daily if we ever felt the need (though will start with 25% 2-3 times a week) - but I don't want the tank to look in any way crowded, so we'll add gradually in species and review as we go.