The "how to breed plecos" is not entirely trivial. Obviously, a lot of what you are asking for depends on the size of the fish itself - a fish that is a foot long will need a bigger tank than one that is about 2" long fully grown.
A fairly generic measure for a cave is around 1.5x the total length of the fish, and a tad taller than the fish is high and width about 1.3x the width of the fish at the widest point. My caves are all home-made, from different materials and different methods - bamboo segments make simple caves, drilling holes in wood is OK up to small-medium size fish, whilst tiles, slate or similar is a useful material for making larger caves. Almost all sizes of caves can be built with PVC/ABS pipes - they may not look very natural, but by all accounts they work. I've used terracotta pots/saucers (for outdoor gardening normally), and use that to make a cave - for example an upside down saucer with one part of the "lip" cut off makes a great cave for Ancistrus and such fish.
Some people take pottery classes at schools or join pottery clubs and such to make clay caves and fire them in a kiln.
There are companies (small and large) that make various forms of caves to buy.
And whilst I've given a pretty precise formula above, I'd say "vary it a bit, and give the fish a choice" - I'm sure we could come up with a formula for the right size bed, sofa, armchair etc, but people still buy different sizes/shapes of those. So give the fish a chance to choose what it likes best.
Tank size is basically the same principle. I use a rule of 4L x 2L x 2L, where L is the size of the fish - that's a minimum size, of course, you can keep smaller fish in the same size tank.
Nearly all plecos can be bred with a pair, but some people seem to have better luck with a larger group - whether that is just because those breeders already had good success with other things, or that there is a good reason to have a larger group. Obviously, with more different fish, the chances of getting them to "hit it off with each other" is more likely.
I'm not a fan of bare bottom tanks, and several members have had bad experiences with only slight lapses in care when cleaning the bottom of a bare-bottom tank, where there appear to be more margin of error for the cleaning with a tank with a layer of fine gravel or sand. I believe several cory breeders have similar stories to tell.
The other part about breeding is to start with something relatively easy - one of the easiest is
. They breed (most of the time) at the drop of a hat, and will is a good start for learning how to care for the fry, etc. There is nothing much worse than getting an expensive/difficult fish to spawn, and then loose all the fry - this will PROBABLY happen even with some practice on an easier to breed/bring up, but hopefully it doesn't take 2-3 tries before you get there.
Which brings me to the biggest factor in success with Plecos: Patience. You need that in lakes of the stuff, not just a few buckets. If at first you don't succeed, try again, and again, and again. May help to change something a bit now and again, but for a large part of it, it's simply "enough time will sort it".
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Mats
