No back up tank right now... To be honest I've actually been banned from keeping any more tanks upstairs since the floor might cave in lol. Looking on-and-off for little tanks but they're all out of my price range. As for the pleco, I've got my 170L (that is, granted, quite overstocked) to move it to in the time being.
Are there any smaller alternatives to corys? I don't think either of my LFS carry dwarf ones, but I'd definitely swap the current cory plan out for dwarf ones if they do.
There’s two types of dwarf/Pygmy Corys. Pandas are also smaller than the rest.
Rosy Loaches are a bit Coryish and small.
I only know of em as the old boy on the corners son knows someone moving house who is breaking down a tank. They’re taking the Livebearers and I’ve been offered everything else!
The three dwarf species of cory (pygmy, habrosus, and hastastus) are softer water cories, needing the GH to be lower than guppies would like it.
Personally, having bronze cories myself, I wouldn't keep a school of them in a 14 gallon long term. The females especially can get really large and chunky, and the ones in my 57 gallon certainly use all of it. But again, that's my personal view, I think a 20 gallon could handle a school of six nicely, but 14 really is a little cramped.
I just think you should really consider what you want this tank to be. Do you want to breed guppies? Or do you want this tank to be fully stocked right away, and stable? A nano tank or a breeding tank? Just go back to basics and really think about what you want from it. Don't think of it as "how many fish can I cram into this sized box?" and start thinking of what you want from it, which species you truly want to keep, and work from there.
If you really want to breed guppies, you want to understock this at the beginning, because you'll have more fry than you know what to do with within a couple of months, and without a spare tank, and your other one being overstocked already, you'll be in trouble in no time. With 12 guppies, even if you go with a male/female ratio of 1:1, that's at least six females churning out 30-40 fry every month. With that, adding a pleco of any size to that tank would be foolish.
Rosey loaches are a great idea for a smaller tank, especially since I think they like harder water, if I remember rightly.