You can get a group of Cories for the bottom. They won't eat algae, but that is just easier to scrape off by yourself. However, if you get these, that means less fish you can keep. I guess you could have 6 Guppies and maybe 3 small Cories. However, you would really need to keep up on the water changes.
I wouldn't get an apple snail, as they have big waste producers and get huge...I've heard of some attacking Bettas, so I definitely wouldn't trust them with fry.
Yes, keep the Guppies in one tank. If you want to raise fry, you definitely are going to want a fry tank.
Only keep maybe 2 females max in the fry tank at once, otherwise you'll just have other females eating the fry.
I'd get some fry food for the first few weeks. That is basically full nutrition without the hassle. Then you can move onto very finely crushed flakes, and small bits of freeze-dried. However, don't feed too much freeze-dried because it can cause them to become bloated and constipated. Also, as I'm sure you know with Betta fry, feed them often! 3 times a day at least, 5-6 is better.
Yes, keep the Guppies in one tank. If you want to raise fry, you definitely are going to want a fry tank.

I'd get some fry food for the first few weeks. That is basically full nutrition without the hassle. Then you can move onto very finely crushed flakes, and small bits of freeze-dried. However, don't feed too much freeze-dried because it can cause them to become bloated and constipated. Also, as I'm sure you know with Betta fry, feed them often! 3 times a day at least, 5-6 is better.