I’m going to start doing the combined Rainbow of Fish and Bottom Dwellers tasks. I’ve never done Frog Breeding at all so will do this alongside it too. What’s the best way to tackle this from the point of view of frog breeding?

It's best to work on the rainbow of fish by themselves. The reward of rainbow rocks will make Bottom Dwellers and Frog Breeding so much easier. Plus, it's best to use forest seeds to deter guppies in order to get 10 of each fish; you won't have enough room to do all this, unless you remove your forest seeds, but them you'll lose out on the gold.

Thank you.

I have done the combined rainbow of fish and bottom dwellers. I liked it because I had to work the old noggin' but I would not recommend it. My advice would be to get your spotting done for all the rainbow of fish and the bottom dwellers before you start. As well, do a round of frog breeding so that you understand how it works. Make sure that you have a number of shroom seeds too(guppies do not need a shroom). And finally, find someone in your same time zone who appears to know what they are doing and only listen to them...because there are different ways to do these challenges.

Here is a really good guide about how to do them together.  The lower ones are definitely easy enough to do together.
http://www.freedomfairies.com/rainbow-fish-and-bottom-dwellers.html

Frog breeding.. since you haven't done any yet, you have to start with a land frog to get spawn.  So the best way would be to do the watery seadragon to plant to 2 pondweed2(you need to remove one then to get used rocks), then attract your 10 snapper, plant coral in the used rocks and get it colored by a red guppy.  Then you go back to land, attract land frog to get spawn, then head back down underwater to attract lobster. If you use superfood, it is enough feeds to get your 10 lobster and a few tadpoles to start the underwater frog breeding.

To add, every 5th landfrog will lay spawn.  Every set of tadpole, froglet and underwater frog you spot in other garden = 1 landfrog.

I’ve done all my spotting already for both the bottom dwellers and rainbow of fish.  I also have a number of tadpoles etc spotted and have ensured I’ve got a large number of UW seeds, so ready to go from that point of view.  My issue I think is that as I don’t understand fully how to do the frog breeding, I’m worried that attracting the Rainbows with pink diamonds is going to start to attract the frogs too and I’ll mess up and waste them.

Oh, and I have enough Supershrooms too!

I know its hard to wrap your head around frog breeding, but once you start, you will better understand it. Until you attract tadpoles and froglets to your garden, you won't have any frogs coming underwater, so you wont have to worry about that for the first color.
Once you got your 1st spawn from a landfrog, you can then start the underwater breeding cycle. That means, every tadpole will return as a froglet, then a frog. You can control it though by using the right plates and/or the frog deterrent, (frogs come only to high plates, froglets only to low plates.)