Bobo! Long time no post aye?
1) I have just started using DIY CO2 and my pH has plummeted. I heard that this isn't something to worry about, because fish aren't actually effected by a changing pH, more a changing kH, is this true? Should I add some more buffering to the tank?
As long as the fish and plants are fine with it, nothing to worry about.
2) Plant set-up. I'm hoping for the glosso to gradually spread until it covers the front of the tank. Then I have to place some plants betweent the ambulia and the glooso - I wanted something mid-height. I was thinking some kind of grass, any ideas?
Hairgrass or lilaeopsis would be nice
3) Fishies. We are going for guppies. Can I keep some kind of small rams in there with them? Maybe some corys? Any other small fish you can recommend (not neons thanks!). I would love some kind of loaches to nail the snails (which are currently Oscar tucker), but am fearful they would munch the guppies' tails?
I think a pair or trio of dwarf cichlids would go nicely in that tank
4) If we got female guppies as well, would they breed of their own volition, or would we have to play around with tank conditions? Could we do all the breeding within this tank, or is it necessary to separate fry from the mothers, and the females from the males? We would only be breeding casually/for fun.
Make sure you have around 2 females for every 1 male. I just caught the fry as I saw them and put them in a floating breeder net inside the main tank.
5) Filter. At the moment I have a crud filter from my old 60l tank. Is filtration important on a densely planted tank? I was thinking it might help with keeping it clear of algae (in some way). Should I go for a canister?
Plants help keep the water chemically clean so it's less important in that aspect than in a non-planted tank. If you have no issues with the current filter then let it be, although more fish could mean more filtration is needed.
HTH and welcome back 8)